STATE  GEOLOGICAL  SURVEY. 

Governor  C.  S.  Deneen,  T.  C.  Chamberlin,  E.  J.  James,  Commissioners 
H.  Foster  Bain,  Director. 


Qualities  of  Clays  Suitable  for 
Making  Paving  Brick  and 
Pryo -Physical  and 
Chemical  Properties 
of  Paving  Brick 
Clays 

BY 

ROSS  C.  PURDY 


[From  Bulletin  No.  9,  pp.  133-278.] 


URBANA 

University  of  Illinois 
1908 


4.  (c  <c  <  'V 


QUALITIES  OF  CLAYS  SUITABLE  FOR  MAKING  PAVING 

BRICK. 

[By  Ross  C.  Purdy.] 


Introduction-. 

Nature  of  the  Problems  Involved — In  Holland  brick  has  been  nsed  for 
street  paving  for  more  than  a  century,  and  in  the  United  States  for  over 
thirty  years.  During  this  period,  ceramics,  or  the  study  of  clay  working, 
has  been  developed  as  a  science  to  such  an  extent  as  to  become,  especially 
in  the  last  decade,  a  prominent  factor  in  the  technical  advance  that  has 
been  made  in  the  various  clay  industries.  The  application  of  pure  sci¬ 
ence,  notably  physics  and  chemistry,  has  solved  a  great  many  practical 
problems,  that  clay  workers  have  met  in  their  endeavor  to  keep  pace 
with  the  ever  increasing  requirements  for  better  quality  and  greater 
adaptations  of  ware. 

1  Ceramics,  or  the  application  of  pure  science  to  clay  working,  has  been 
developed  chiefly  along  two  lines:  first,  the  applications  of  mechanics 
to  the  evolution  of  methods  of  winning  raw  materials,  and  manufactur¬ 
ing  of  wares;  second,  the  application  of  physical  and  chemical  prin¬ 
ciples  to  the  selection  and  mixing  of  clays  and  minerals. 

Along  these  two  lines  ceramics  has  attained  its  greatest  development 
in  the  pottery,  floor  and  wall  tile,  and  kindred  industries  where  white 
burning-clays  and  pure  minerals  are  blended  in  the  manufacture  of 
wares.  The  compounding  of  the  white  ware  mixtures  and  the  processes 
for  their  manufacture  can  now  be  said  to  have  emerged  from  the  strictly 
empirical  stage  and  to  have  reached  a  degree  of  perfection  that  cor¬ 
rectly  merits  the  designation  of  "applied  science.” 

In  the  brick,  tile  and  kindred  industries  which  use  more  complex 
clays — clays  that  naturally  contain  sufficient  fluxes  to  produce  the 
requisite  degree  of  hardness  in  the  burned  ware — ceramics  has  de¬ 
veloped  principally  along  the  first  line,  the  application  of  mechanical 
principles  to  the  processes  of  manufacture.  In  this,  ceramics  has 
kept  abreast  of  the  demands.  Along  the  second  line,  the  application 
of  pure  science  to  the  determination  and  control  of  the  properties  of 
tyody  mixtures,  but  very  little  progress  has  been  made.  In  this,  science 
has  been  baffled  by  the  complexity  of  the  mineral  mixture  which  nature 
has  compounded  and  man  calls  clay. 

Complexity  of  properties  is  the  natural  result  of  complexity  of  min¬ 
eral  composition.  If  this  complexity  of  mineral  composition  resulted 
simply  in  variation  of  chemical  constituent,  the  problem  would  be  com¬ 
paratively  simple,  but  the  physical  properties  of  each  of  the  several 
u/iinerals  are  nearly,  if  not  equally,  as  potent  factors  in  complicating 
the  problems,  as  the  chemical. 

\  133 


p  2439t> 


134 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


The  science  of  ceramics  is  making  rapid  progress  in  the  solution  of 
these  problems;  but  today  the  question  of  what  physical  and  chemical 
properties  raw  clay  must  possess  that  it  might  be  suitable  for  use  in  the 
manufacture  of  paving  brick  is  still  unanswerable.  Chemical  analysis 
alone  is  not  a  safe  criterion  by  which  to  decide  this  and  similar  ques¬ 
tions,  for,  as  can  be  shown  in  nearly,  if  not  all,  geological  survey  re¬ 
ports  on  clays,  the  analyses  of  clays  that  are  known  to  be  suitable  for 
paving  brick  have  their  counterparts  in  analyses  of  building  brick  clays. 
Indeed  in  range  of  variation  in  chemical  constituents,  these  two  types 
of  clays  overlap  one  another  to  a  very  large  extent.  There  are  possibly 
one  or  two  characteristics  in  the  chemical  composition  of  paving  brick 
clays  that  are  not  common  to  those  used  for  building  brick,  and  yet  no 
fixed  rule  has  been,  or  so  far  as  the  writer  can  perceive,  can  be,  laid 
down  at  present,  by  which  to  identify  paving  brick  clay  by  chemical 
analysis. 

Physical  tests  on  green  or  unburned  clay,  so  far  as  is  now  known, 
would  not  lead  one  any  nearer  the  possibility  of  fairly  judging  a  pav¬ 
ing  brick  clay  than  would  chemical  analysis.  Possibly  an  exception 
should  be  made  of  determinations  of  fineness  of  grain.  Plasticity, 
tensile  strength,  bonding  power,  slaking  properties,  etc.,  are  .found  to 
vary  widely  in  different  paving  brick  clays,  so  that  no  dependence  can 
be  placed  upon  any  of  them,  taken  alone.  The  determination  of  fine¬ 
ness  of  grain,  however,  does  give  a  negative  test  that  seems  to  be  of 
some  value. 

Fine  grained  clays,  as  will  be  seen  later,  have  not  proved  to  be  good 
paving  brick  clays.  It  cannot  be  said,  however,  that  all  coarse  grained 
clays  are  good  paving  brick  clays.  Indeed,  although  evidence  is  lack¬ 
ing,  there  is  no  obvious  reason  for  believing  that  any  hard  and  fast 
rule  can  at  present  be  laid  down  in  regard  to  either  fine  or  coarse 
grained  clays. 

When  the  history  of  a  few  paving  brick  plants  in  various  parts  of 
this  country  reveals  the  fact  that  experienced  paving  brick  manufactur¬ 
ers  have  so  misjudged  a  deposit  of  clay  as  to  erect  an  extensive  plant 
upon  a  particular  site  and  soon  find  that  they  must  abandon  the  idea 
of  attempting  to  make  any  other  than  a  building  brick,  it  must  be  in¬ 
ferred  that  even  a  burning  test  as  ordinarily  conducted  by  ceramic 
engineers,  surveys  and  brick  machine  manufacturers  likewise  often 
gives  evidence  that  is  untrustworthy.  By  what  means  then  can  the 
suitability  of  a  clay  for  paving  brick  purposes  be  ascertained? 

It  was  with  hopes  of  obtaining  evidence  upon*  this  problem  that  the 
Survey  undertook  a  study  of  the  properties  of  the  clays  and  burned 
bricks  of  several  of  the  leading  paving  brick  manufactories  in  the 
middle  west,  together  with  several  samples'  of  clays  from  various  parts 
of  this  state,  that  are  not  now  being  used  for  paving  brick  manufacture. 

For  many  years  scientists  have  been  devising  methods  with  which’ 
to  determine  the  cause  and  effect  of  the  various  properties  of  clay,  but 
they  have  not  made  much  progress.  For  instance,  the  reason  why 
kaolin  and  a  ball  clay,  having  similar  chemical  composition  and  size 
and  apparently  character  of  grain,  should  differ  so  widely  in  plasticityj, 


purdy]  QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BKICK.  135 

is  still  an  open  question.  The  refractoriness  of  a  clay  is  still  incal¬ 
culable  from  analytical  data,  although  exhaustive  researches  have  been 
made  to  determine  the  pyro-chemical  effect  of  inorganic  acids  and  bases, 
singly,  collectively,  and  in  mixtures,  with  standard  clays  and  com-, 
pounds.  While  from  these  pyro-chemical  studies  it  has  been  shown 
that  the  fluxing  power  of  the  bases  is  roughly  proportional  to  their  mole¬ 
cular  weight,  and  that  the  several  acids  operate  in  a  definite  manner,  so 
that  synthetical  mixtures  can  be  made  with  assurance  that  each  compon¬ 
ent  will  operate  in  a  given  manner,  and  that  the  resultant  effect  of  the 
mixtures  will  in  general  be  as  presupposed,  similar  natural  mixtures, 
known  as  clays,  exhibit  properties  that  are  in  the  large  majority  of 
cases  entirely  contradictory  to  those  of  synthetical  mixtures,  due  no 
doubt  to  differences  in  the  physical  properties  of  the  minerals  as  well  as 
to  variation  in  mineral  content. 

Many  theories  have  been  advanced  concerning  the  geological  history  of 
clays,  and  general  statements  can  be  made  as  to  the  probable  conditions 
that  cause  the  breaking  down  of  the  parent  rock,  the  character  of  the  resi¬ 
dual  debris,  the  agencies  sorting  and  transporting  this  debris,  and  the 
conditions  under  which  it  can  be  deposited  in  different  grades  of  fineness 
and  purity.  Geologists  can  state  with  considerable  accuracy,  the  effect  of 
vegetable  growth  and  of  ground  water,  the  cause  for  the  precipitation  of 
salts  from  solutions,  the  cementing  value  of  various  compounds  under 
different  conditions,  etc.  They  can  establish  the  faot  that  there  is  a  cycle 
of  rock  decomposition,  residual  deposition,  and  rock  formation  going  on 
constantly  en  masse ,  as  well  as  in  the  small  grains  of  which  clay  and  soils 
are  composed.  Yet,  after  all,  neither  geologists  nor  chemists  are  able 
to  determine  the  exact  stage  of  breaking  down  or  building  up,  nor  the 
exact  combination  of  several  ingredients  existing  in  a  clay  at  the  time  of 
examination.  It  certainly  seems  patent  that  until  we  can  determine  the 
exact  mineralogical  condition  and  chemical  aggregation  of  a  given  clay 
it  will  be  impossible  to  use  the  analytical  data  obtained  by  ordinary  phy¬ 
sical  and  chemical  tests  as  ground  for  predictions  concerning  its  probable 
pyro-chemical  behavior. 

In  the  process  of  any  chemical  analysis  known  to  the  writer,  the 
character  and  exact  identity  of  the  clay  as  a  whole,  as  well  as  its  con¬ 
stituent  parts,  are  destroyed  by  the  disintegration  or  unlocking  of  the 
natural  combinations,  making  an  exact  or  complete  determination  of  the 
chemical  conditions  originally  present,  a  mere  supposition.  In  fact 
all  we  know  or  can  learn  from  a  study  of  the  origin  and  mode  of  forma¬ 
tion  of  clays,  and  of  the  alterations  in  their  composition  constantly  go¬ 
ing  on  under  varying  conditions,  as  well  as  by  attempts  to  unlock  the 
combinations  or  separate  the  ingredients  by  chemical  methods  is,  that  we 
are  arresting  the  changes  of  transition  in  the  clay  from  one  state  to  an¬ 
other,  but  are  not  able  to  ascertain  the  forms  or  conditions  existing  at 
that  time.  From  these  considerations  it  should  be  plain  that  two  samples 
of  clays  having  similar  origin  and  chemical  constitution,  may  differ  rad¬ 
ically  in  their  mineralogical  make-up.  The  kind,  size  and  composition  of 
the  several  minerals  affect  so  materially  the  pyro-chemical  properties  of 
the  clay  as  a  whole,  that  until  mineralogists  can  find  means  of  determin- 


136 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


ing  the  kind,  quantity  and  relative  size  of  the  several  mineral  ingredi¬ 
ents,  ceramists  cannot  predict,  even  with  a  fair  degree  of  accuracy,  the 
behavior  of  a  clay  in  burning. 

Several  of  the  so  called  physical  properties  of  raw  clay  may,  however, 
be  measured  and  their  effect  on  the  behavior  of  the  clay  described.  In 
this,  physical  conditions  of  the  clay,  such  as  hardness,  fineness  of  grain, 
plasticity,  etc.,  are  by  custom  regarded  as  properties. 

The  properties  of  clays  may  be  classified  under  three  general  heads : 
Physical,  chemical,  and  pyro-chemical. 

PHYSICAL  PROPERTIES. 

Introduction. 

Ceramists  have  tested  clays  by  all  the  means  that  have  been  suggested 
to  them.  Many  of  the  tests  have  proven  fruitless,  and  not  a  few  now  in 
use  are  of  doubtful  value. 

The  following  physical  tests  on  raw  clays  were  made  by  the  State 
Survey : 

1.  Specific  gravity  of  the  clay,  or  mineral  aggregate.  2.  Porosity  of  a 
dry,  unburned  brick  made  from  “stiff  mud.”  3.  Drying  behavior.  4.  Shrink¬ 
age.  5.  Tensile  strength.  6  Fineness  of  grain.  7.  Water  of  plasticity. 
8.  Plasticity. 

Specific  Gravity. 

REAL  AND  APPARENT  SPECIFIC  GRAVITY. 

The  determination  of  the  specific  gravity  of  a  clay,  if  made  at  all, 
should  be  so  conducted  that  the  result  would  be  a  composite  of  the  speci¬ 
fic  gravities  of  the  several  minerals  that  make  up  the  clay  mass.  If  the 
specific  gravity  of  a  lump  be  taken  as  a  whole,  unless  the  mass  be  so 
thoroughly  saturated  that  each  grain  becomes  surrounded  by  the  saturat¬ 
ing  medium,  it  would  vary  with  all  kinds  of  irregularities  incident  to  the 
processes  of  formation  or  manufacture.  The  first  method  would  give 
what  is  known  as  true  specific  gravity,  while  the  second  would  give  only 
an  apparent  value. 

The  writer  can  see  no  value  in  finding  the  apparent  specific  gravity, 
for,  as  a  means  of  detection  of  any  working  property ,  it  is  absolutely 
valueless.  The  true  specific  gravity  may  have  direct  value  as  indicating 
some  working  property  of  the  clay,  but  if  it  has,  the  fact  has  not  been 
demonstrated.  The  data  for  the  true  specific  gravity  can,  however,  be 
used,  as  will  be  demonstrated  later,  in  the  analysis  of  some  of  the  changes 
that  take  place  in  drying  and  burning,  and  serves  as  a  check  on  the 
accuracy  of  some  of  the  other  data. 

METHODS  OF  DETERMINATION. 

The  true  specific  gravity  of  the  clays  included  in  this  report  was  ob¬ 
tained  by  three  methods :  By  Seger  volumeter,  using  unburned  bricks ; 
by  pycnometer;  by  chemical  balance,  using  unburned  bricks. 

Determination  by  Seger  s  Volumeter. — Seger’s  volumeter  was  used  in 
the  determination  of  the  volume  shrinkage,  porosity  and  specific  gravity 
on  the  several  clays,  as  noted  in  Table  I  . 


Table  I — Results  of  Physical  Tests  on  Green  Bricks. 


PURDY]  QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK.  137 


Per  cent 
Porosity. 

Percent 
variation. .. 

0'#ooMNO)HHoooooainioocDot-N®aHooootDifl+Nt'C'ini-«o.o 

Average . 

oSSS^SosNH^NNMifliOO^OOOCWOa^OOQNiOaOOWOSWOSlO 

a  g  g  d  g  g  n  s  a  si  s'  a  a  s’  s'  a  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s  s'  s' 

Minimum  . .. 

00[-00!D»NOrt(»NScONO«Oit'^iON«HOOOi05H01tOfflNOOtOH» 

s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s  s'  s'  s  s'  s  s  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s' 

Maximum.. .. 

t'H00NNSiHO!00'nSininriHMD-00'#NinCC05Hl£IMt'M0iOO00OlOC' 

s'  s'  s'  s  s'  s'  s'  s'  s'  s'  s'  a  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s'  s' 

.1 ■at' 

Apparent  Specific 
Gravity. 

Percent 
variation. . . 

Average . 

Minimum  . .. 

2  °°.  ®  os  us  c-  o  o  os  th  ?o  cm  oo  us  co  -<*  co  co  io  os  ec  oo  co  id  o  cm  i-h  o»  c-  os  cm  co 

io£ioioSDcDg33gio§cococo3cDio3co8coiocD2io82£ioSg3i®2 

NNNNNNNNNMNNWNNNNNNNNNNMNNNNNNNNNNN 

S  2  S3  S  ©  2  S3  g  5;  2  S3  5  g  2  3  S3  2  8  3  S3  8  8  8  g  2  2  S  3  S3  S  S3  8  S  S  2 

CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  (M  CM  CM  CM  CM  CM  CM*  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM  CM 

Maximum.... 

3  g  g  8  3  8  P  £  g  8  g  8  g  3  5  5  g  8  5  3  8  g  8  g  2  8  g  3  8  g  3  8  8  8  3 

CM  CM  CM  CM  Cl  CM  CM  Cl  Cl  CM  CM  Cl  Cl  Cl  Cl  Cl  Cl  CM  Cl  Cl  Cl  Cl  Cl  CM  Cl  Cl  CM  CM  CM  CM  CM  CM  CM  CM  CM 

Per  cent  Linear 
Shrinkage. 

Percent 

variation... 

Average . 

CO  1  CD  +05  CMCMOOOOO^^OO 

©oococo©eM©©©'^coc-+iocMTHin©eM© 

133. 

70. 

68 

73 

129 

tl 

95 

75 

37 

73 

44 

48 

93 

§2©CMioi-l05r-ia5aOWCOC010 

t-CM10CO©CMCO©©lOCMCOt-00©C-aOCM'-i<© 

HOONCOH^CONOWCOCOCOH 

(N^^CvOCOCOCOlOHlO^^CON^WCOI>I>Tt- 

Minimum  . . . 

OOClCOOOOOn-OCDOOOOOOCM 

O  CM  t— l  CM  O  OO  CM  t— 1  ©  ID  CM  CM  CM  t— 1 

WOMOONONNXNOONCO^NCDCDCDCO 

CMCOCOCMCMCMCM'^OCO'^^COrHiO'^lOiOiOCO 

Maximum.. . . 

00C0O'*<0000"*i<©00CM-**''#-«tlC0 

OOOCO©COOOCOOO©aO-^ICOCMOOaO-*<-^'COOO-*t< 

CM^CO^CM^^COrHt^^^^frJ 

^'^lO^COCO^iOCOCOlOlOlCCOC^t'-tr-OOOOiO 

Per  cent  Volume. 
Shrinkage. 

Percent 
variation. .. 

OO-^O-OOC^COC^iOrHlOOOOiiOiO 

©coc-co  •  t—  .ooin®roiH»®cD  -oc-ao*o 

050100  ;©  jC-CMC-N^CD©^  ;«HOO 

Average . 

cmS2SSoo3SS»oS3S 

051-H©1-I  -05  :  kO  W  CO  OO  T*  c-  00  ^  :iOO'<*'cd 

CDHHHi°2ait>WHHHHCO 

PSS°>  ;SS8P 

Minimum  . .. 

s  ®  04  05  ®  s'  d  s'  ®  " 

11.44 

12.6 

13.5 
8.00 

5.76 

11.0 

6.98 

13.7 

13.6 

14.1 
9.54 
7.17 

20.7 

16.2 

16.8 
19.9 

10.07 

Maximum.... 

co  8  co  2  co  t-  g  eo  oi  S  05  g  g 

+ d  d  s  *°*  s'  s*  °°"  °°' s  s'  s'  s'  ® 

13.71 

14.0 

14.3 

10.31 

6.16 

11.9 

7.9 

14.8 
14.0 

14.7 
10.20 

8.70 

21.7 

16.7 
20.0 
21.1 

11.9 

Per  cent  Hygroscopic 
Water. 

Percent 
variation. . . 

13.9 
9.2 
4.1 

13.0 

19.5 

25.9 

4.7 
20.2 

13.4 
8.0 

11.5 

8.7 
21.3 

10C-CO'#lOCM©OlOCOi-ICMlO©©00©OOlOC- 

^'s'g's's'^'^'ds'd^'^'^'s'g's'ss's'd 

Average 

3SS  :lsSS23Sgsl 

NHN  jdrtHHONHWNd 

-TiiCMi-ii-ii-ieMCMT-<T-iooeMeMcci-icoeMeMoocMi-i 

Minimum  . . . 

1.88 

1.55 

2.64 

6.87 

1.08 

1.65 

1.66 
0.75 
2.19 
1.74 
4.93 
2.14 
0.693 

g  8  2  8  5  3  8285  8  g  5  8  ©  3  8  2  g  8 

^  CM  rH  t-I  t-I  W  CM  rH  tH  CM  CM*  CM  CM  rH  CO  CM*  CM*  CO*  »-h*  tH 

Maximum.. .. 

2.16 

1.70 

2.54 

0.99 

1.32 
2.15 
1.74 
0.81 
2.50 
1.88 
5.52 

2.33 
0.86 

S  g  2  g  £  g  iS  g  8  8  g  £  S  8  ^  t-  S  8  8  S 

CM*  CM  t-H  (M  «M  CM  H  h*  CO*  CM  CM  CO*  rH  ^  CM  CM  ^  CM  t-H* 

Per  cent  Water 
of  Plasticity . 

Per  cent 
variation . . . 

co  P  05  c-  -«*  iq  oq  oo  ©  5  c-  th  cm  t-  ©  co  cm  co  oo  cm  cm  co  o  co  o  oo  co  c-  cm  cm 

'^■^‘CMCOlrtCOCMCMCDOOCOOSCDCO-^'-HOCOOCM*— It— (CM'^lO'^lOOO'^CDC-COCM'^CD 
i-l  -i— 1  l—l  i-l  CM  i-l 

Average . 

C5paoSooS'#'!HCDSeooocoa5CMCD'#ococMcDt-aoio-*j<W'#eMCO'^'co©-^<oo 

d  s’  s  s’  s  d  d  d  s  s'  s’  s’  s’  s’  s  d  s  s'  s'  s’  s  d  d  d  s’  d  s  s  s’  s'  s'  s'  s'  d  d 

Minimum.. .. 

CDCMlOOOOOOO'iieMl-lt-ggcDlO'^IOOeMT-lCOT-lCMlO'^lDOSOiili-HOOOi-HOOOOSlO 

PSSSSSSPSSPSSSSSSS2S22SPSPSSSSSSS82 

Maximum  ... 

eqcD©a5io->*a5coa5'SiSoe-©eMcoa5©co-'#©c-a5©oocOi-HCOio©eococoooeo 

s'  d  d  s’  s'  d  d  d  s'  §  s'  d  s'  d  d  d  d  d  s’  s'  s'  d  d  d  s’  d  d  s'  s'  d  s’  s'  s’  d  s’ 

Sample  number . 

Sc-w^iD^Voo'^S^SS^S^^dcM'co’SSV^+GY^SSSasa^ 

Kiln  Letter . 

:0L'rO£KNJt 

138 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


The  accuracy  of  the  tests  in  the  volumeter  is  shown  in  Table  I  by  the 
percentage  of  variation  volume  shrinkage,  specific  gravity  and  porosity. 
These  variations  are  very  small  considering  the  conditions  under  which 
the  determinations  were  made.  * 

The  specific  gravity  of  any  substance  is  the  ratio  of  the  weight  of  that 
substance  to  the  weight  of  an  equal  volume  of  some  substance  taken  as  a 
standard.  In  the  metric  system  distilled  water  at  4°C.  is  taken  as  the 
standard.  At  this  temperature  a  cubic  centimeter  of  distilled  water 
weighs  one  gram.  Therefore,  when  using  this  system,  volume  and 
weight  of  water  may  be  interchanged,  i.  e.,  1  c.c.=l  gram. 

With  this  understanding  the  formula  by  which  the  specific  gravity  of 

a  clay  can  be  obtained  could  be  expressed  as; 

W  „ 

—  ==  bp.  gr. 


where  W=dry  weight  in  grams  and  V=volume  in  cubic  centimeters.  If 
the  porosity  of  the  brick  has  been  determined  the  formula  for  the  specific 
gravity  could  be  written : 


w 

Y  (100— P) 


bp.  gr. 


Where  W=dry  weight  as  before,  Y=volume  of  the  brick  in  cubic 
centimeters  and  P=percentage  porosity. 

It  will  be  noted  by  comparing  the  specific  gravities  in  Tables  I  and 
II,  that  those  obtained  by  the  volumeter  are  lower  than  those  obtained  by 
the  pycnometer.  This  can  be  accounted  for  perhaps  by  the  operator’s  in¬ 
ability  completely  to  saturate  a  brick,  that  is,  to  fill  all  the  pore  spaces 
with  oil  without  resorting  to  the  use  of  a  suction  or  vacuum  pump  to  re¬ 
move  all  the  air  from  the  pores  so  that  oil  could  enter.  If  the  air  is  not 
entirely  exhausted  it  will  pass  through  the  oil  very  slowly,  requiring  a 
period  extending  over  several  weeks  in  which  to  esape.  In  ordinary  labor¬ 
atory  practice  sufficient  time  can  not  be  given  to  permit  the  complete  es¬ 
cape  of  the  included  air.  In  the  porosity  and  specific  gravity  tests  here 
reported,  no  attempt  was  made  to  fill  the  pores  completely.  The  bricks 
were  simply  soaked  in  coal-oil  for  48  hours,  with  one  face  exposed  at  the 
level  of  the  surface  of  the  oil.  This  incompleteness  of  saturation  under 
these  conditions  is  shown  by  the  difference  in  the  specific  gravity  as  de¬ 
termined  by  the  volumeter  and  pycnometer. 

Determination  by  Pycnometer. — A  pycnometer,  or  specific  gravity 
bottle,  as  it  is  often  called,  is  a  small  flask  of  known  capacity,  usually 
25  to  100  c.c.  When  filled  up  to  a  given  mark  with  air-free  water  at 
normal  room  temperature,  its  weight  is  noted.  The  flask  is  then  partly 
emptied,  a  known  weight  of  clay  added,  and  the  whole  carefully  boiled  to 
exclude  all  the  entrapped  air,  then  cooled,  filled  up  to  the  mark  and 
weighed.  By  the  formula,  weight  of  dry  sample  (a)  plus  weight  of 
bottle  filled  with  cold  air-free  water  (b)  minus  weight  of  bottle  filled 
with  sample  and  water  (c),  or  a+b — c,  will  give  the  weight  of  water 
having  the  same  volume  as  the  sample  or  true  total  volume  of  the  clay 
particles.  Knowing  the  dry  weight  and  true  volume  of  the  grains,  their 
composite  specific  gravity  is  readily  calculated  by  the  formula  (dry 
weight  -T-  volume). 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


139 


This  may  be  illustrated  by  the  following  calculation :  . 

Weight  of  bottle  filled  with  water  =  143.22. 

Dry  weight  of  sample  =  3.41. 

Weight  of  bottle  +  sample  +  water  required  to  fill  to  mark  =  145.35. 
143.22  +  3.41  —  145.35  =  1.28  total  volume  of  the  particles. 

3.41  -f-  1.28  =  2.68  specific  gravity  of  the  sample. 

In  the  following  table  will  be  found  specific  gravity  of  the  clays  by  the 
pycnometer  method. 


TABLE  II. 


I 

II 

Average 

K  1  Alton,  Ill  . 

2.666 

2.664 

2.665 

K  2  St.  Louis,  Mo . 

2.602 

2  527 

2.564 

K  3  Albion,  Ill . 

2.688 

2.684 

2.686 

K  4  Springfield,  Ill . 

2.667 

2.668 

2.667 

K  5  Edwardsville,  Ill . 

K  6  Galesburg,  Ill . 

2.676 

2.661 

2.626 

2.664 

2.651 

2.663 

K  7  Streator,  Ill . 

2.643 

2.63 

2.636 

K  8  Veedersburg,  Ind . 

2.693 

2.685 

2.689 

K  9  Crawfordsville,  Ind  . 

2.701 

2.703 

2.702 

K  10  Terre  Haute,  Ind . . 

2.683 

2.689 

2.686 

K  11  Brazil,  shale . 

K  12  Brazil,  fire  clay . 

2.667 

2.671 

2.669 

K  13  Clinton,  Ind . 

2.682 

2.708 

2.695 

K  14  Western  Brick  Co . 

2.633 

2.646 

2.639 

K  15  Barr  Clay  Co.,  Streator,  111 . 

2.719 

2.713 

2.716 

R  l  Nelson ville,  O . 

2.633 

2.632 

2.633 

R  2  Portsmouth,  O . 

2.719 

2.712 

2.715 

R  3  Canton  Imperial . 

2.655 

2.656 

2.655 

R  4  Canton  Royal . 

2.720 

2.722 

2.721 

S  1  Moberly,  Mo . 

2.643 

2.646 

2.643 

S  2  Kansas  Citv,  Mo . 

2.717 

2.716 

2.717 

F  1  Danville  Brick  Co . 

2.708 

2.710 

2.709 

H  24  Carbon  Cliff,  fire  clay . 

2.660 

2.654 

2.657 

H  17  LaSalle,  Ill . 

2.608 

2.591 

2.599 

H  16  Peoria,  Ill . 

2  700 

2.690 

2.690 

2  695 

H  18  Sterling,  Ill . 

2.653 

2.671 

H  23  Carbon  Cliff,  shale . 

2.628 

2.624 

2.626 

H  21  Galena,  Ill . 

2.718 

2.715 

2.717 

H  20  Savanna,  Ill . 

2.718 

2.715 

2.717 

H-II  ToDeka.  Kan . 

2  683 

2.685 

2.684 

L-II  Lawrence,  Kan . 

2.702 

2.707 

2.705 

I-II  Casey,  Kan . 

2.668 

2.676 

2.672 

J-II  Pittsburg,  Kan . 

2.699 

2.697 

2.698 

B-II  Atchison,  Kan . 

2.666 

2.668 

2.667 

G-II  Coffeyville,  Kan . 

2.704 

2.707 

2.706 

Determination  with  Chemical  Balance. — The  dry,  saturated  and  imT 
mersed  weights  of  briquettes  were  determined  by  using  a  chemical  bal¬ 
ance.  For  this  it  was  found  that  briquettes  of  the  size  %"x%"x2%" 
could  be  used.  Obviously  the  larger  the  briquette  the  more  nearly  true 
will  be  the  determined  specific  gravity.  Sizes  larger  than  that  given, 
however,  cannot  be  used  to  advantage  on  the  ordinary  chemical  balance. 
This  method  was  used  for  but  a  small  number  of  samples. 

The  briquettes  were  dried  to  constant  weight  in  an  air  bath  at  120 °C. 
cooled  in  a  dessicator  and  their  dry  weight  obtained  as  rapidly  as  possible. 
After  weighing,  the  briquettes  were  immersed  in  clarified  coal-oil  with 
one  face  above  the  level  of  the  oil.  After  standing  thus  for  20  to  24 
hours,  they  were  placed  under  a  bell  jar  and  the  air  kept  exhausted  for 
fifteen  minutes,  it  having  been  found  in  previous  work  that  this  treat¬ 
ment  was  sufficient  to  attain  nearly  complete  saturation.  The  briquette 
was  then  suspended  by  a  silk  thread  from  the  beam  of  a  chemical  bal- 


140 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


ance  and  its  saturated  weight  noted.  A  breaker  partially  filled  with  oil 
was  then  so  placed  that  the  briquette  could  swing  clear  and  be  com¬ 
pletely  immersed.  In  this  manner  the  immersed  weight  of  the  briquette 
was  obtained. 

By  the  formula  then  of  dry  weight  (D)  divided  by  (dry  weight  (D) 
minus  suspended  weight  (S)  )  orD-f  (D — S),  the  specific  gravity  of 
the  material  in  the  briquette  was  readily  obtained. 

The  comparative  accuracy  attained  in  the  determination  of  the  speci¬ 
fic  gravity  of  clay  by  these  three  methods  may  be  seen  in  the  table  fol¬ 
lowing. 

Table  III. 


Volumeter- 

Average. 

Pycnometer- 

Average. 

Chemical 

Balance. 

Average  of 
three 

determina¬ 

tions. 

Max. 

Min. 

K  1 . 

2.54 

2.66 

2.614 

2.616 

2.615 

K  14 . 

2.60 

2.64 

2.69 

2.57 

2.65 

K  4 . 

2.58 

2.67 

2.63 

2.58  * 

2.61 

It  is  evident  from  this  table  that  the  essential  fault  in  the  first  and 
third  method  of  determining  specific  gravities  lies  in  the  fact  that  the 
brick  was  not  completely  saturated,  and  therefore,  gave  low  specific 
gravities.  The  closeness  in  agreement,  however,  suggests  that  by  the  use 
of  extra  precaution  in  the  saturation  of  the  bricks  the  specific  gravities 
of  the  clays  could  be  made  quite  accurately  by  either  of  these  two 
methods. 


Porosity. 

DEFINITION". 

The  percentage  of  porosity  expresses  the  relation  between  the  volume 
of  pore  space  and  the  combined  volume  of  the  particles  ‘of  which  the 
clay  is  composed.  It  is  the  ratio,  in  terms  of  volumes,  of  void  spaces  to 
solid  particles.  If  determined  on  an  unburned  brick  it  would  measure 
the  degree  of  consolidation  of  the  mass. 

METHOD  OP  DETERMINATION. 

The  porosity  of  an  unburned  clay  mass  may  be  determined  directly  by 
two  methods :  first,  by  use  of  the  Seger  volumeter ;  second,  by  the  use  of 
a  chemical  balance,  and  indirectly,  or  by  calculation  on  basis  of  the 
pycnometer  specific  gravity  determination. 

To  obtain  percentage  of  porosity  by  either  of  the  direct  methods,  the 
briquette  or  lump  must  be  dried  to  constant  weight,  and  the  dry  weight 
obtained,  then  saturated  in  kerosene  and  the  saturated  weight  obtained. 
The  difference  between  the  saturated  and  dry  weights  is  obviously  the 
weight  of  petroleum  that  is  required  to  fill  the  pores.  This  weight 
divided  by  0.8,  the  density  of  the  oil,  gives  the  equivalence  of  oil  in 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK. 


141 


terms  of  water.  Thus  far,  therefore,  the  actual  amount  of  pore  space  in 
the  brick  in  terms  of  water  by  weight  is  known.  If  the  metric  system 
has  been  used  throughout,  this  amount  of  water  by  weight  is  equivalent 
to  its  amount  by  volume,  since  one  cubic  centimeter  of  water  at  room 
temperature  weighs  practically  one  gram. 

Complete  saturation  of  a  lump  of  unburned  clay  even  with  kerosene, 
for  which  clay  seems  to  have  a  peculiar  physical  attraction,  cannot  be 
obtained  without  resorting  to  the  use  of  a  vacuum  pump.  Standing  in 
oil  for  48  hours  is  not  sufficient  to  cause  complete  saturation,  as  has  been 
shown  on  proceeding  pages  by  the  specific  gravity  so  obtained,  as  well 
as  by  the  discrepancy  between  the  directly  measured  and  the  calculated 
porosity  as  given  in  table  IV,  page  144. 

In  obtaining  the  dry  and  saturated  weights,  the  two  direct  methods 
are  alike.  The  data  for  actual  volume  of  the  pores  thus  obtained  are, 
however,  of  no  value  in  themselves,  and  cannot  become  of  value  until 
calculated  to  parts  of  100  unit  volumes  of  the  whole  brick.  For  this, 
it  is  more  practical  to  determine  the  volume  of  the  whole  mass,  i.  e., 
pores  plus  solid  particles.  It  is  in  the  determination  of  the  volume  of 
the  mass  that  the  two  direct  methods  above  mentioned  are  differen¬ 
tiated. 

First  method ,  Volumeter. — After  complete  saturation,  the  brick  is 
placed  in  the  volumeter  and  its  volume  determined  in  cubic  centimeters. 
W— S 

By  the  formula  100  ( - )  where  W=weight  of  oil  taken  up  by  the 

Y 

brick,  S=the  specific  gravity  of  the  oil,  and  V=the  volume  of  the  brick, 
there  is  expressed  the  part  of  100  unit  volumes  of  the  brick  as  a  whole 
which  consists  of  open  pore.  In  other  words,  it  is  the  percentage  por¬ 
osity. 

By  referring  to  Table  I  it  will  be  noted  that  the  percentage  of  varia¬ 
tion  in  the  porosity  determination  was  relatively  small.  Since  the  data 
given  in  Table  I  represents  determinations  made  on  60  bricks  of  each 
clay  47.5  as  the  maximum,  0.6  per  cent  as  the'  minimum,  and  11.5  as 
the  average  percentage  variation,  is  considered  as  being  excellent. 

These  percentages  of  variation  in  results  are  not  surprising  in  view  of 
the  fact  that  an  error  of  lcc.  in  determination  of  volume,  or  an  error 
of  1  gram  in  obtaining  either  the  dry  or  saturated  weight,  makes  a  dif¬ 
ference  of  0.3  in  the  porosity. 

It  is  obvious,  therefore,  that  when  the  dry  weight  of  the  bricks  are 
obtained  they  must  be  absolutely  dry,  i.  e.,  oven  dried  at  120° C.  so  as 
to  expel  all  of  the  hygroscopic  water.  This  was  not  done  in  obtaining 
the  data  given  in  Table  I. 

Second  method ,  Chemical  Balance. — When  the  porosity  of  the  brick 
is  determined  on  a  chemical  balance  the  volume  of  the  briquette  is 
found  by  the  apparent  loss  of  weight  of  the  briquette  when  suspended 
in  the  oil.  The  briquette  appears  to  lose  weight  when  thus  suspended, 
and  this  loss  of  weight  is  equivalent  to  the  weight  of  a  quantity  of  oil 


142 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


equal  to  that  of  the  briquette.  This  method  was  used  by  Dr.  E.  R. 
Buckley  in  the  test  on  the  Wisconsin  clays,  and  the  porosity  calculated 
by  him  using  the  formula: 

looi  ^ — w  D)  Sp.  gr. — _==  r  cen^  p0r08ity. 

I(W-D)  Sp.  gr.+D 

In  this  formula  W=saturated  weight;  D=dry  weight;  and  Sp.  gr.  the 
composite  specific  gravity  of  the  clay  particles,  as  calculated  from  dry, 
saturated,  and  suspended  weights  of  the  briquette. 

This  formula,  however,  can  be  simplified  by  substituting  for  the  value 
of  D  in  the  denominator  its  value  in  terms  of  the  Sp.  gr.  and  suspended 
weight  (S)  as  given  in  the  formula  for  specific  gravity  where  D=D 
(Sp.  gr.)— S  (Sp.  gr.). 

rw— d) 

The  Buckley  formula  then  simplifies  to  the  expression  100  k - 1= 

[w-sj 

porosity.  This  formula  holds  true  no  matter  what  liquid  is  used  in  the 
saturation  of  the  brick,  so  long  as  the  same  liquid  is  employed  in  ob¬ 
taining  the  suspended  weight. 

The  method  is  accurate  but  very  slow  and  tedious,  unless  it  is  carried 
out  with  small  pieces  on  the  jolly  balance. 

If  a  jolly  balance  is  used  in  this  determination,  the  weight  of  the  bri¬ 
quette  or  piece  must  not  exceed  that  which  would  stretch  the  spring  be¬ 
yond  its  elastic  limit.  If  any  other  than  a  light  weight  spring  is  used  the 
difference  between  the  several  readings  will  not  be  sufficient  to  permit  of 
very  accurate  determination.  This  method  was  used  in  the  determina¬ 
tion  of  the  rate  of  vitrification,  which  will  be  described  under  the  gen¬ 
eral  heading  of  “Pyro- Chemical  Tests/5  so  will  not  be  discussed  in  de¬ 
tail  at  this  time. 

Third  Method ,  Calculation. — It  has  been  noted  that  the  specific  grav¬ 
ity  of  the  powdered  clay  by  the  pycnometer  method  is  uniformly  higher 
than  that  calculated  from  data  obtained  on  the  green  bricks.  It  has 
also  been  noted  that  this  difference  between  the  specific  -gravities  is  due 
to  the  incomplete  saturation  of  the  brick.  Since  the  formula  for  speci¬ 
fic  gravity  is:  Dry  weight  (W)  divided  by  the  combined  volume  of  the 
W 

particles  (V)  or  — =Sp.  Gr.,  the  true  volume  of  the  particles  in  the 
Y 

brick  can  be  obtained  bv  the  formula :  Dry  weight  divided  by  the  pycno- 

D 

meter  specific  gravity,  or - =Yol.  Then  the  volume  of  the  whole 

Sp.  Gr. 

brick  (Vb)  minus  the  volume  of  the  clay  particles  (Vc)  would  give  the 
volume  of  the  pore  spaces  (Vp),  or  Vb — YC==YP’.  To  obtain  the  frac¬ 
tional  amount  of  pore  space  in  a  brick,  the  volume  of  the  pores  (Yp) 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


143 


yp  yb _ yc 

must  be  divided  by  the  volume  of  the  brick:  or  — .  But  since - = 

yb  yb 

f  Vc]  .  W 

—  we  have  100  ]  1 - J=per  cent  pore  space  where  Vc= - 

Vb  l  VbJ  Sp.  Gr. 

The  economy  and  accuracy  in  determining  porosity  by  this  method 
lies  in  the  fact  that  it  is  not  necessary  to  saturate  the  brick  and  obtain 
the  saturated  weight.  It  is  obvious,  therefore,  that  the  bricks  would 
either  have  to  be'  partially  saturated  or  covered  with  a  thin  coating  of 
paraffin  and  their  volume  determined  in  a  volumeter.  Without  a  volu¬ 
meter  this  method  cannot  be  used. 

If  the  specific  gravity  has  been  determined  by  the  pycnometer  method 
and  a  volumeter  is  not  accessible,  the  porosity  is  best  calculated  by  the 
Buckley  formula.  In  this,  however,  complete  saturation  of  the  brick 
must  be  assured,  and  the  true  specific  gravity  of  the  clay  particles  used. 

Neither  the  Buckley  method  nor  the  indirect  method  here  proposed 
is  usable  on  any  other  than  a  green  or  unburned  lump  of  clay.  For  the 

[W-Dl 

porosity  of  a  burned  lump  or  briquette,  the  formula  100  ■{ - V  is 

[W— SJ 

the  only  one  that  will  give  accurate  results,  as  will  be  shown  under  the 
discussion  of  Pyro-Chemical  and  Physical  Properties  of  Clays. 

In  the  following  table  are  given  porosity  data  obtained,  first,  by  the 
usual  volumeter  method  without  taking  into  account  the  hygroscopic 
water;  second,  by  the  indirect  method  described  above,  without  taking 
into  account  the  hygroscopic  water,  and  third,  by  the  indirect  method  on 
a  basis  of  absolute  dryness  of  the  bricks.  The  percentage  of  increase  of 
porosity  obtained  in  the  second  and  third  instance,  over  that  obtained 
in  the  first  is  also  shown. 


144 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[bull.  no.  9 


Table  IV — Showing  the  percentage  of  errors  in  the  Seger  Volumeter 
method  of  determining  porosity  as  customarily  executed. 

Calculated  results  are  by  the  indirect  method. 


Sample 

and 

Brick  Number. 

Dry  weight 
in 

grains— 

Volume  in  cubic  centimeter. 

Porosity  by  volumeter  on 

air  dried  briquettes. 

Porosity  by  calculation  on 

air  dried  briquettes. 

Percentage  increase  in  poro¬ 

sity  by  calculations  over 
that  by  volumeter. 

Porosity  by  calculations  on 

oven  dried  briquettes. 

Percentage  increase  by  cal¬ 

culation  on  overdried 
briquettes  over  that  by 

volumeter  on  air  dried 

briquettes. 

Air  dried. 

Oven  dried. 

K  3—34 . 

571.5 

556.4 

300.2 

23.0 

29.12 

26.61 

31.0 

34.78 

K  3-37 . 

572.5 

558.0 

297.8 

24.0 

28.43 

18.45 

30.24 

26.00 

K  3-39 . 

601.3 

587.7 

311.5 

23.8 

28.13 

18.19 

29.76 

25.04 

K  5-13 . 

644.9 

639.3 

329.5 

25.6 

26.17 

2.22 

26.81 

4.73 

K  5—15 . 

647.1 

640.7 

330.6 

25.4 

26.17 

3.03 

26.89 

5.87 

K  5-17 . 

658.4 

652.7 

337.4 

25.5 

26.39 

3.49 

27.03 

6.00 

K  7-  1 . 

607.4 

596.6 

327.7 

27.3 

29.69 

8.76 

30.94 

13.34 

K  7-3 . 

586.5 

574.0 

312.1 

26.3 

•28.71 

9.16 

30.23 

14.94 

K  7-5 . 

600.4 

587.5 

323.4 

26.9 

29.57 

9.93 

30.08 

11.82 

K  8—19 . 

663.7 

652.4 

336.6 

24.3 

26.67 

9.75 

27.92 

14.89 

K  8—24 . 

631.2 

620.7 

320.7 

24.1 

26.81 

11.24 

28.03 

16.31 

K  8—31 . 

621.5 

611.7 

316.3 

24.1 

26.93 

11.75 

28.08 

16.51 

K10 —  1 . 

545.5 

533.2 

284.6 

25.0 

28.64 

14.56 

30.25 

21.49 

K10—  2 . 

539.9 

523.5 

281.6 

24.7 

28.62 

15.87 

30.79 

24.66 

K10—  4 . 

532.2 

520.0 

278.8 

25.7 

.28.94 

12.60 

30.56 

18.91 

K13-44 . 

622.4 

607  9 

323.6 

26.6 

28  63 

7.63 

32.10 

20.67 

K 13-46 . 

600.7 

587.8 

310.3 

26.6 

28.17 

5.90 

29.71 

11.69 

K 13-53 . 

619.5 

605.9 

322.0 

28.4 

28.61 

6.27 

30.18 

7.40 

H18—  1 . 

639.2 

625.3 

315.6 

21.8 

24.17 

10.87 

25  82 

18.44 

H18-  3  . . 

637.0 

624.0 

314.3 

22.1 

24.12 

9.14 

25.67 

16.16 

H18—  5 . 

642.9 

629.6 

317.1 

21.7 

24.09 

11.01 

25.67 

18.30 

H20 —  1 . 

583.0 

567.4 

300.1 

23.0 

28.49 

23.87 

30.40 

32.18 

H20 —  3 . 

588.2 

576.3 

303.2 

23.0 

28.59 

24.30 

30.03 

30.56 

H20 —  4 . 

579.4 

563.5 

296.6 

23.0 

28.09 

22.13 

30.06 

30.70 

H24 —  1 . 

601.0 

591.4 

291.4 

17.5 

22.38 

27.88 

23.62 

34.97 

H24 —  3 . . 

650.5 

640.0 

314.4 

17.6 

22.13 

25.74 

23.38 

32.84 

H24 —  5 . 

600.5 

591.5 

292.9 

18.3 

22.84 

24.81 

23.99 

31.09 

R  3—1 . 

687.1 

672.9 

344.3 

23.2 

24.83 

7.03 

26.39 

13.75 

R  3—3 . 

667.0 

653.9 

336.3 

23.0 

25.30 

10.00 

26.76 

16.35 

R  3-5 . 

690.9 

678.7 

348.5 

23.1 

25.33 

9.65 

26.65 

15.37 

R  4—26 . 

705.6 

689.2 

348.7 

20  3 

25.64 

26.31 

27.36 

34.78 

R  4-29 . 

678.1 

663.0 

337.0 

21.2 

26.05 

22.88 

27.70 

30.66 

R  4-36 . 

716.1 

697.8 

352.0 

20.3 

25.24 

24.34 

27.15 

33.74 

S  1-1 . 

575.5 

545.7 

309.5 

21.2 

29.70 

40.09 

33.34 

52.55 

S  1—3 . 

573.0 

541.5 

307.2 

20.0 

29  48 

47.40 

33.36 

66.80 

S  1-5 . ;.... 

565.5 

540.0 

303.8 

21.5 

29.62 

37.77 

33.80 

57.20 

L-II-11 . 

599.0 

584.2 

322.6 

26.0 

31.35 

20.57 

33.03 

27.04 

L- 1 1-13 . 

612.2 

591.4 

326.7 

23  9 

30.72 

28.53 

33.10 

38.40 

L-II— 15 . 

602.3 

584.4 

324.8 

25.6 

31.44 

22.81 

33.47 

30.74 

G-II-55 . 

708.7 

702.6 

349.7 

22.5 

24.67 

9.64 

25.32 

12.53 

G-II-57 . 

705.3 

696.8 

347.8 

22.7 

25.03 

10.26 

25.93 

14.23 

G- IT-58 . 

712.3 

704.0 

349  6 

22.7 

24.68 

8.27 

25.55 

12.56 

The  data  in  table  IV  shows  the  inaccuracy  of  the  nsual  method  of  de¬ 
termining  the  porosity  in  dried  clay  wares.  It  has  been  stated1  that  three 
to  six  honrs  is  sufficient  to  saturate  with  oil  unburned  briquettes  that 
measure  3x1  inches.  Forty-eight  hours  was  therefore  considered 

ample  time  in  which  to  saturate  a  brick  that  cubically  was  about  eight 
times  as  large.  From  the  fairly  close  agreement  in  the  specific  gravities 
as  determined  by  the  pycnometer  and  the  volumeter,  it  was  thought  that 
the  briquettes  had  been  fairly  well  saturated.  Such,  however,  was  evi¬ 
dently  not  the  case. 


Iowa  Geological  Survey,  Vol.  14,  p.  18. 


PURDY]  QUALITIES  OF  CLAYS  FOR  MAKING  PAVING  BRICK. 


145 


RELATION  OF  RATE  OF  ABSORPTION  TO  POROSITY. 

Aside  from  exposing  the  irregularities  in  our  method  of  analysis,  this 
data  gives  evidence  of  the  lack  of  relation  of  total  porosity  and  rate  of 
absorption  in  the  green  or  unburned  bricks.  Since  all  the  bricks  were 
subjected  to  the  same  oil  immersion  treatment,  it  must  follow  that  clays 
differ  in  the  rate  at  which  they  can  be  saturated,  and  that  this  rate  is 
not  wholly  a  function  of  porosity. 

The  writer  is  not  aware  of  tests  ever  having  been  made  to  investigate 
this  property,  which  we  may  call  “absorption  ratio,"  but  its  significance 
in  connection  with  the  drying  behavior  of  clays  is  obvious. 

Johnson1  has  said,  “Obviously,  too,  the  quantity  of  liquid  in  a  given 
volume  of  soil  affects  not  only  the  rapidity,  but  also  the  duration  of 
evaporation.  The  following  table,  by  Schubler,  illustrates  the  peculiar¬ 
ities  of  different  soils  in  these  respects.  The  first  column  gives  the  per¬ 
centages  of  water  absorbed  by  the  completely  dry  soil.  In  these  experi¬ 
ments  the  soils  were  thoroughly'  wet  with  water,  the  excess  allowed  to 
drip  off,  and  the  increase  of  weight  determined.  In  the  second  column 
are  given  the  percentages  of  water  that  evaporated  during  the  space  of 
four  hours  from  the  saturated  soil  spread  over  a  given  surface." 

TABLE  V. 


Percent. 

1 

Per  cent. 

Quartz  sand .  . 

25 

88.4 

Gypsum . 

27 

71.7 

Fine  sand . 

29 

75.9 

Slaty  marl . 

34 

68  0 

Clay  soil  (60  %  clay) . 

40 

52.0 

Loam . 

51 

45.7 

Plough  land . 

52 

32.0 

Heavy  clay  (80 1  clay) . 

61 

34.9 

Pure  gray  clay . 

70 

31.9 

Fine  carbonate  of  lime . 

85 

28.0 

Garden  mould . 

89 

24.3 

Humus . 

(81 

25.5 

Fine  carbonate  of  magnesia . 

256 

10.8 

“It  is  obvious  that  these  two  columns  express  nearly  the  same  thing 
in  different  ways.  The  amount  of  water  retained  increases  from  quartz 
sand  to  magnesia.  The  rapidity  of  drying  in  the  air  diminishes  in  the 
same  direction." 

Johnson  affirms2  that  “these  differences — (in  the  imbibing  power  of 
clays) — are  dependent  mainly  on  the  mechanical  texture  or  porosity  of 
the  material."  That  Johnson’s  statement,  when  applied  to  unburned 
bricks,  is  incorrect,  is  shown  by  the  data  in  table  IY.  That  there  are 
other  factors  affecting  the  difference  in  rate  of  absorption  and  evapora-  - 
tion  in  different  clays  is  quite  evident. 

Value  of  the  Porosity  Determination  on  Raw  Clay  Lump. — It  has 
been  contended  at  various  times  in  ceramic  literature  that  a  porosity  de¬ 
termination  on  a  raw  lump  of  clay  would  give  evidence,  concerning  such 
properties,  as  slaking,  weathering,  amount  of  water  required  to  de- 


1  Loc.  Cit.,  p.  18. 

2  How  Crops  Feed,  p;  175. 

—10  Gr 


146 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


velop  plasticity,  etc.,  and  thus  indirectly  the  shrinkage.  Such  claims 
have ‘never  been  based  on  data,  nor  are  they  substantiated  by  the  data 
secured  by  this  Survey.  As  will  be  shown  in  later  paragraphs,  neither 
data  nor  sound  reason  would  warrant  such  statements. 

Value  of  the  Porosity  Determination  on  Green  Brick. — Before  the 
value  of  knowing  the  porosity  of  a  green  brick  can  be  discussed  it  is  neces¬ 
sary  to  show  the  correlation  of  that  property  with  those  which  it  affects. 
If  porosity,  fineness  of  grain,  and  drying  behavior  are  in  any  degree  re¬ 
lated  functions,  curves  plotted  from  data  should  show  such  relations. 
Such  a  relation  is  shown  in  Fig.  4  where  there  seems  to  be  an  inverse 
ratio  between  fineness  of  grain  in  surface  or  loose  grained  clays,  and  the 
porosity  of  the  green  or  unburned  brick.  The  data  from  which  this 


Fig.  4.  Curve  showing  relation  between  porosity  and  fineness  of  grain.  (From  data  of  Beyer 
and  Williams,  Iowa  Geol.  Surv.,  Vol.  14,  p.  123.) 

curve  was  plotted  was  obtained  from  the  work  of  Beyer  and  Williams1. 
The  surface  factor  was  calculated  from  their  data  by  the  method  given 
on  page  113.  The  porosity  data  and  calculated  surface  factor  are  as 
follows : 


TABLE  IV. 


Porosity. 

Surface  factor. 

Palp  Rrirt  Cn  . 

18.14 

136.40 

Gethmaun  Bros . ■ . 

22.43 

114.38 

L.  C.  Besley  (bottom) . 

24.03 

119.98 

L.  C.  Besley  (middle) . 

L.  C.  Besley  (top) . 

25.30 

29.77 

100.48 

74.77 

This  reciprocal  relation  between  fineness  of  grain  and  porosity  could 
be  taken  as  evidence  in  proof  of  the  close  relation  of  fineness  of  grain 
and  porosity  of  the  green  brick  to  drying,  shrinkage  and  other  properties 
that  are  peculiar  to  wares  manufactured  from  fine  but  loose-grained 
clays  or  mixtures.  The  writer  hesitates,  however,  to  affirm  the  truth  of 

1  Iowa  Geol.  Surv.,  Vol.  XIV,  p.  123. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


147 


such  a  relation  from  evidence  obtained  on  a  few  samples  of  a  single  type 
of  clay.  Observation  of  the  working  behavior  of  boulder  clays  in  build¬ 
ing1  brick  manufacture  does  not  lead  one  to  believe  in  such  an  exact  rela- 
tion. 

In  the  manufacture  of  bricks  by  the  ordinary  dry  process — where  there 
is  present  only  from  6  to  12  per  cent  of  mechanical  water,  and  the  grains 
of  the  clay  are  not  surrounded  by  slippery  media  that  permit  the  particles 
to  slide  easily  and  freely  upon  one  another — the  clay  cannot  be  formed 
into  as  compact  a  mass  as  when  there  is  sufficient  water  present  to  permit 
of  manufacture  by  the  stiff  mud  process.  If  dry  pressed  bricks  be  formed 
in  a  hammer  machine,  or  press,  where  the  brick  is  subjected  to  repeated 
blows  by  a  heavy  hammer,  the  clay  particles,  even  though  nearly  dry, 
would  be  forced  over  one  another  until  the  mass  assumes  a  much  closer  or 
denser  structure  than  is  possible  by  the  ordinary  dry  press  process.  The 
difference  in  structure,  and  its  consequent  effect  on  the  burning  properties 
of  dry  press  bricks  manufactured  by  these  two  methods  in  the  St.  Louis 
district  is  more  evident  than  the  difference  between  the  structure  of  the 
dry  and  stiff  mud  or  stiff  mud  and  soft  mud  bricks. 

When  the  grains  are  made  to  lie  closer  together,  either  by  strong 
force,  or  by  a  lighter  force  supplemented  by  a  floating  medium,  better 
opportunity  is  offered  for  the  grains  to  fuse  with  one  another.  This  is 
shown  nicely  by  the  fact  that  hammered  brick  can  be  burned  in  the 
colder  parts  of  a  kiln  to.  a  degree  of  hardness  that  is  equal  to  and  often 
exceeds  the  hardness  of  a  brick  made  from  the  same  clay  by -the  dry 
press  method  and  burned  in  the  hotter  portions  of  the  kiln. 

In  the  illustrations  just  cited,  the  difference  in  the  burning  properties 
of  bricks  made  from  the  same  clay  by  different  processes,  has  been  used 
as  a  means  of  noting  that  porosity  of  green  brick  is  not  wholly  a  func¬ 
tion  of  size  of  grain.  It  is  clear  that  it  is  also  very  largely  a  function 
of  process  of  manufacture. 

While  there  may  be  some  relation  between  the  size  of  grain  of  loose 
or  soft  clays  and  the  porosity  of  the  brick  manufactured  from  them,  it 


Fig.  5.  Diagram  showing  the  relation  between  porosity  of  green  shale  brick  and  the  absolute 
fineness  of  grain.  (From  data  given  in  Table  I.) 


148 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


is  still  doubtful  if  a  similar  relation  can  be  observed  in  the  hard  rock¬ 
like  fossil  clays,  such  as  shales,  where  the  mineral  particles  are  so  cement¬ 
ed  as  to  very  stubbornly  resist  separation  by  the  crushing  force  of  dry 
pan  mullers  as  well  as  the  disintegrating  influence  of  the  water  used  in 
pugging. 

In  Fig.  5  are  shown  curves  plotted  from  data  obtained  with  shales  in 
the  same  manner  as  the  data  for  surface  clays  in  Fig.  4.  The  porosity 
is  taken  from  Table  I,  and  the  surface  factor  from  Table  VIII. 

The  clay  from  which  these  shale  brick  were  made  had  been  crushed  to 
pass  through  a  dry  pan  and  then  screened.  In  the  laboratory  they  were 
known  as  dry  pan  samples.  These  “dry  pan  samples”  were  then  in  the 
same  state  of  mechanical  subdivision  as  the  clay  used  by  the  manu¬ 
facturer. 

In  making  the  bricks  from  which  the  data  in  Table  I  were  obtained, 
considerable  time  was  expended  in  pugging  or  wedging  the  clays  by 
hand,  first  in  a  large  bulk,  and  later  in  quantities  just  sufficient  for  one 
brick.  The  operator  batted  a  quantity  of  clay  that  would  make  approxi¬ 
mately  60  bricks  tyz”  on  a  plaster  top  table  until  it  was  as 

compact  as  he  could  make  it.  Then  by  use  of  a  trowel  in  some  instances 
and  a  wire  in  others,  he  cut  off  from  .the  large  mass  a  quantity  sufficient 
to  fill  the  die  of  the  press.  This  smaller  piece  was  again  thoroughly 
wedged  by  hand  until  all  air  blebs  had  been  worked  out  and  the  whole 
took  on  the  shape  of  a  compact  loaf.  This  loaf  was  then  placed  in  the 
die,  using  care  to  see  that  it  cleared  the  sides  so  as  to  prevent  a  shearing 
off  of  any  portion  of  the  loaf  on  the  edge  of  the  die  when  the  plunger 
descended.  The  loaves  were  pressed  into  bricks  on  a  slow  screw  tile  press, 
so  that  the  clay  did  not  receive  much  compression,  but  yet  sufficient  to 
cause  it  to  flow  in  shreds  up  around  one  side  or  another  of  the  plunger. 
From  this  flowage  of  the  clay  past  the  plunger,  together  with  the  unusual 
amount  of  wedging  by  hand,  it  was  considered  that  the  clay  had  been  sub¬ 
jected  to  a  treatment  that  was  approximately  comparable  to  the  pugging 
it  would  have  received  in  the  factory,  so  that  the  data  as  given  in  Table 
I  show  approximately  the  physical  structure  except  for  lamination  that 
would  be  developed  on  a  regular  manufacturing  basis. 

It  is  commonly  known  by  paving  brick  manufacturers  that  some  shales 
require  inordinate  pugging  before  they  develop  sufficient  plasticity  to 
permit  the  production  of  a  perfect  bar  in  the  die  of  the  brick  machine. 
In  fact  it  is  not  uncommon  to  see  a  battery  of  two  pug-mills  through 
which  the  clay  must  pass  before  it  enters  the  brick  machine  proper.  In 
the  brick  machine  the  clay  receives  further  pugging  before  it  issues  as  a 
bar  from  the  die.  It  is  also  not  uncommon  to  hear  the  manufacturers 
claim  that  they  cannot  pug  clay  sufficiently  unless  they  use  hot  water. 
Not  all  manufacturers  have  to  resort  to  this  extra  care  in  the  pugging 
process,  for  some  shales  develop  plasticity  with  sufficient  readiness  to 
allow  them  to  emerge  from  the  first  pug-mill  in  a  workable  condition. 
This  same  difference  in  the  working  property  of  the  various  shales  was 
perhaps  more  noticeable  in  the  laboratory  than  in  the  factories. 

This  difference  in  the  working  properties  of  shales  is  considered  to  be 
due  to  the  fact  that  the  grains  of  clay  are  cemented  by  substances  that 
differ  in  their  solubility  in  water.  It  is  now  well,  known  that  soils  and 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICKS.  149 

clays  contain  soluble  salts  that  are  adsorbed  by,  or,  to  use  a  more  homely 
expression,  smeared  over  the  particles,  and  are  not  easily  extracted  by 
water.  It  has  been  learned  by  experiment  that  clays  can  take  on  or 
adsorb  soluble  salts  from  solutions  and  so  retain  these'  salts  in  their  sub- 
microscopic  pores  that  they  cannot  again  under  ordinary  conditions  be 
dissolved  from  the  clay. 

The  amount  of  water  used  in  the  pugging  of  shales  is  not  sufficient  to 
dissolve  or  loosen  all  of  the  cementing  salts  in  a  clay  even  by  continued 
pugging,  so  that  at  best,  only  a  portion  of  the  clay  particles  are  separated 
from  one  another,  but  the  manufacturers  must  continue  the  pugging 
until  a  sufficiently  large  number  of  grains  are  separated  to  form  a  slippery 
medium,  by  virtue  of  which  the  unslaked  or  undisintegrated  bundles  of 
particles  can  slip  past  one  another  freely  enough  to  permit  a  flowage  of 
the  mass  under  pressure.  The  difficulty  encountered  by  manufacturers 
in  breaking  down  the  cementing  bond  in  shales  is  increased  many-fold 
when  an  attempt  is  made  to  disintegrate  a  clay  into  its  ultimate  grains, 
as  is  done  in  mechanical  analysis.  The  data  for  texture  or  size  of  grain 
used  in  plotting  the  curves  in  Figs.  4  and  5  were  obtained  by  mechanical 
analysis  and  are  supposed  to  represent  the  subdivision  of  the  clays  into 
their  ultimate  particles.  While  it  is  comparatively  easy  to  obtain  separa¬ 
tion  of  the  particles  in  loose-grained  clays  in  the  laboratory  and  in  the 
factory,  it  is  obvious  that  it  is  not  possible  to  obtain  a  similar  separation 
of  the  particles  of  the  hard  rock-like  clays  in  the  factory,  and  very  diffi¬ 
cult  to  obtain  much  more  than  an  approximation  of  ultimate  subdivision 
in  the  laboratory.  It  is  owing  to  this  indefinite  degree  of  solution  of  the 
natural  bond  in  pugging  that  we  have,  in  the  case  of  shale  bricks,  a  dis¬ 
cordant  relation  between  the  porosity  of  the  brick  and  the  fineness  of 
grain,  shown  in  Figs.  2  and  5,  as  contrasted  with  the  semingly  concord¬ 
ant  relation  in  the  case  of  the  loess  bricks,  as  shown  in  Fig.  4. 

Although  a  porosity  determination  on  a  green  brick  may  not  be  of 
value  as  direct  evidence  of  the  so-called  “working  properties”  of  a  clay, 
it  can  be  shown  that  it  is  of  indirect  value,  in  that  the  data  can  be  used 
as  the  basis  of  many  interesting  and  valuable  calculations.  For  practical 
demonstration  of  the  commercial  possibilities,  or  exposition  of  the  work¬ 
ing  properties  of  a  clay,  the  writer  has  failed  to  find  wherein  porosity 
data  on  unburned  bricks  separately  considered  are  of  use. 

Fineness  of  Grain. 

DEFINITION. 

By  fineness  of  grain  or  texture  of  a  clay  is  meant  the  size  of  its  min¬ 
eral  particles.  Experimental  evidence  indicates  that  variation  in  grain 
controls  many  of  the  physical  and  pyro-chemical  properties  exhibited  by 
clays.  Plasticity,  shrinkage  in  drying  and  burning,  tensile  strength, 
drying  properties,  rate  of  oxidation,  rate  of  vitrification,  toughness  of 
burned  ware,  and  finally,  to  some  extent  pyrometric  value  of  the  clay, 
are  all  influenced  by  fineness  of  grain. 


150 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


The  grains  of  many  clays  are  so  cemented  that  they  resist  separation 
in  the  ordinary  png-mill  or  blnnger.  When  two  or  more  particles  are 
thus  cemented  they  operate  as  a  unit  in  their  influence  upon  plasticity, 
tensile  strength,  drying  behavior,  etc.  This  accounts,  in  part,  for  many 
of  the  apparent  exceptions  to  the  general  rules  deduced  from  experiment¬ 
al  evidence,  for,  in  the  usual  methods  applied  for  determining  fineness 
of  grain,  special  effort  is  made  to  separate  the  particles  completely. 
This  raises  the  question  whether  separation  of  the  particles  should  be 
carried  to  such  extremes  when  attempting  to  trace  direct  relations  be¬ 
tween  fineness  of  grain  and  the  physical  properties  developed  in  the 
process  of  manufacture  of  clay  into  wares.  On  this  point,  however,  we 
have  no  direct  evidence,  except  perhaps  as  shown  in  Figs.  4  and  5,  so  the 
question  will  have  to  remain  unanswered  for  the  time  being. 

It  is  known,  however,,  that  it  would  be  well-nigh  impossible  to  determ¬ 
ine  how  far  a  mechanical  separation  of  the  particles  should  be  carried 
in  the  laboratory  to  make  the  test  comparable  to  the  separation  effected 
in  the  pug-mill,  wet  pan,  or  blunger.  For  this  reason  it  would  seem  as 
though  the  most  useful  data  concerning  texture  or  fineness  of  grain  can¬ 
not  be  obtained  by  the  present  method  of  analysis. 


MEANS  OF  EXPRESSING  FINENESS  OF  GRAIN. 

If  all  the  particles  of  clay  were  considered  as  being  spheres  or  cubes 
their  super ficiaT areas  would  be  inversely  proportioned  to  their  diameters. 
The  following  calculations  show  this  to  be  true  in  regard  to  the  sphere : 

D3 

Volume  of  a  sphere  is  equal  to  Pi  — ;  then  if  D  and  d  are  the  diameters 

6 


PiD3  Pid3 

of  two  spheres  their  volumes  would  be  proportional  as - : - 

6  6. 

The  number  of  spheres  required  to  equal  in  volume  a  standard  unit 
PiD3  6  6 

volume  would  be  1  - or  -  in  the  one  case,  and  -  in  the 


Pi  D3 


Pi  d3 


other.  Since  the  surface  of  a  sphere  of  each  size  is  equal  respectively  to 
Pi  D2  and  Pi  d2  the  total  surface  area  of  a  collection  of  spheres,  having  a 

6  6 

total  volume  equal  to  unity,  would  be  in  each  case - x  D2  and - 

Pi  D3  Pi  d3 


6  6 

x  d2  or - and - respectively.  The  combined  areas  of  each  group 

PiD  Pid 

rf  spheres  occupying  the  same  volume  but  having  different  diameters  are, 
therefore,  inversely  proportional  to  their  diameters.  This  proportional 
relation  of  the  surface  of  the  particles  in  the  several  groups  is  taken  as 
the  surface  factor  of  the  respective  groups,  and  the  sum  of  these  as  the 
surface  factor  of  the  clay. 

Cushman1  has  shown  the  error  involved  in  thus  taking  the  mean  of  the 
extreme  diameters  in  a  given  group.  According  to  data  given  by  Cush- 


1  Air  Elutriations  of  Fine  Powders,  Jour.  Am.  Chem.  Soc.,  Vol.  XXIX,  No.  4,  p.  589, 

April  1907. 


PURDY] 


QUALITIES  QF  CLAYS  FOR  MAKING  PAVING  BRICKS. 


151 


man,  a  mechanical  analysis  of  the  separate  groups  would  show  a  predom¬ 
inance  (77  to  87  per  cent  in  Cushman  data)  of  the  finer  particles  of  that 
group.  That  the  mean  diameter  obtained  as  described  above,  is  not  a  true 
mean  of  the  diameters  of  the  particles  in  a  group,  is  obvious.  The  error 
thus  involved  cannot,  however,  be  obviated  without  a  much  more  exten¬ 
sive  subdivision  of  the  groups  than  is  possible  under  ordinary  conditions. 
It  needs  no  mathematical  demonstration  to  make  clear  that,  theoretically, 
the  more  extensive  the  analysis,  the  more  accurate  would  the  results  be. 
It  needs  but  a  short  experience  with  the  mechanical  analysis  by  any  of 
the  hydraulic  methods,  to  learn  that,  practically,  the  more  extensive  the 
analysis  is  made,  the  larger  will  be  the  operating  errors.  In  making  a 
mechanical  analysis  one  must  choose  between  the  Scylla  and  Charybdis  of 
these  errors  and,  naturally,  will  decide  in  favor  of  that  one  which  involves 
the  making  of  the  fewest  determinations. 

In  this  report  the  mean  of  the  extreme  diameters  of  each  group,  irre¬ 
spective  of  the  distribution  by  number  according  to  their  volume,,  of  the 
particles  within  the  respective  groups  is  taken  as  representing  the  diam¬ 
eter  of  the  group.  The  mean  diameter  of  each  group  and  total  surface 
factors  for  the  clays  here  reported  are  shown  in  Table  Y. 

VALUE  OF  DETERMINATION  OF  FINENESS  OF  GRAIN. 

As  before  stated,  fineness  of  grain  is  the  probable  cause  of  several  of 
the  other  properties  exhibited  by  clays.  Since  fineness  of  grain  is  the 
cause,  and  the  other  properties,  in  a  large  sense,  the  effects,  the  true 
significance  of  this  determination  can  be  best  discussed  by  dealing  sep¬ 
arately  with  the  properties  induced  by  size  of  grain. 

Numerical  Results. — In  Table  YII  is  given  the  per  centage  by  weight 
of  calcined  materials  in  each  of  the  several  groups  according  to  sizes  of 
particles. 


TABLE  VII. 


Sample  Number. 

Hygro¬ 

scopic 

water. 

Com¬ 

bined 

water. 

Percentage  Amount  by  Weight  of  Particles, 
Grouped  According  to  Diameters. 

Total. 

1  M.  M. 

l-.IM.M. 

.1-.01. 

.01-. 001. 

.001-0.0 

K  1 . 

0.47 

5.73 

6.92 

6.19 

54.24 

22.92 

7.87 

104.36 

K  2 . 

1.03 

3.77 

0.96 

1.14 

63.75 

18.04 

13.33 

102.04 

K  3 . 

0.97 

6.90 

1.42 

1.47 

54.38 

23.03 

12.00 

100.18 

K  4 . 

1.68 

5.43 

1.30 

1.66 

46  47 

27.76 

19.34 

104.68 

K  5 . 

1.10 

5.60 

5.91 

1.04 

58.01 

21.04 

9.69 

102.41 

K  6 . 

0.70 

4.76 

1.14 

1.74 

63.17 

23.49 

7.62 

102.65 

K  7  . 

0.67 

5.46 

1.14 

3.42 

58.82 

24.45 

9.75 

103.73 

K  8. . 

1.20 

7.40 

8.76 

6.55 

45.95 

22.48 

8.26 

100.61 

K  9 . 

0.83 

3.32 

11.03 

1.49 

63.80 

13.79 

6.53 

100.83 

K  10 . 

1.74 

5.52 

0.85 

2.09 

22.96 

40.72 

23.93 

97.84 

K  11 . 

1.78 

8.32 

3.02 

2.69 

40.44 

33.76 

11.13 

101.15 

K  12 . 

1.48 

8.66 

3.64 

2.21 

38.03 

35.83 

12.13 

102.00 

K- 13 . 

0.92 

6.13 

1.60 

0.79 

44.70 

38.80 

11.37 

104.33 

K  14 . 

0.66 

5.08 

13.66 

5.76 

41.24 

24.34 

8.14 

98.91 

R  1 . 

1.33 

7.69 

1.76 

5.90 

36.59 

36.23 

12.97 

102.49 

R  3 . 

1.10 

5.29 

10.92 

5.84 

50.67 

20.52 

9.93 

104.30 

R  4 . 

0.82 

6.11 

9.47 

2.54 

47.69 

26.78 

8.98 

102.43 

G-II . 

1.35 

4.28 

12.67 

2.38 

50.78 

20.19 

12.80 

104.48 

I-II . 

2.01 

4.35 

4.14 

3.62 

44.37 

25.24 

17.53 

101.30 

H  18 . 

1.69 

14.95 

12.14 

12.14 

24.57 

20.88 

17.60 

103.99 

H  20 . 

1.98 

11.97 

1.00 

1.86 

39.73 

29.73 

16.69 

102.79 

H  21 . 

1.63 

12.26 

0.19 

0.50 

22.50 

39.21 

26.39 

102.71 

H  23 . . 

2.48 

7.71 

1.60 

2.56 

28.11 

38.20 

21.89 

102.57 

152 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS, 


[bull.  no.  9 


In  Table  VIII  will  be  found  the  same  data  with  the  hygroscopic  water 
eliminated  and  the  chemical  water  distributed  over  the  various  groups 
proportionally  to  the  amount  belonging  to  each,  as  determined  by  their 
loss  on  ignition. 

TABLE  VIII. 


Sample 

Number. 

1  mm 
mean 
diam. 
1.25. 

1mm 

mean 

diam. 

0.5. 

0.1-.01 

mean 

diam. 

0.05. 

.01- .001 
mean 
diam. 
0.005. 

.001-0 

mean 

diam. 

0.0005. 

Total. 

Surface 

factor 

K  1 . 

7.27 

6.53 

56.07 

24.86 

9.76 

104.51 

256. 

K  2 . 

1.07 

1.23 

66.24 

19.63 

.  13.90 

102.09 

331. 

K  3 . 

1.50 

2.41 

57.15 

25.14 

13.96 

100.18 

341. 

K  4 . 

1.40 

1.74 

48.87 

29  41 

22.24 

103.68 

514. 

K  5 . 

6.38 

1.46 

60.57 

22.93 

11.43 

102.80 

287. 

K  6 . 

1.24 

1.83 

65.83 

25.98 

7.77 

102.66 

221. 

K  7 . 

1.35 

3.75 

60.87 

25.89 

11.81 

103.69 

300. 

K  8 . 

9.66 

6.90 

48.46 

25.40 

10.05 

100.50 

262. 

K  9 . 

11.39 

1.55 

65.50 

14.72 

7.63 

100.80 

195. 

K  10 . 

1.06 

2.42 

24  62 

44.29 

25.52 

97.91 

604. 

K  11 . 

5.36 

3.76 

43.74 

35.45  . 

12.94 

101.26 

339. 

K  12 . 

4.49 

2.88 

40.51 

38.82 

15.32 

102.04 

403. 

K  13 . 

1.82 

1.35 

46.74 

41.73 

13.15 

104.80 

356. 

K  14 . 

14.23 

6.31 

42.75 

26.03 

9.67 

98.99 

254. 

R  1 . 

2.16 

6.51 

38.70 

39.32 

15.53 

102.25 

397. 

R  3 . 

11.69 

6.30 

52.90 

21.60 

11.79 

104.29 

291. 

R  4 . 

10.15 

2.84 

49.32 

29.13 

10.85 

102.31 

275. 

I- II . 

4.64 

3.81 

45.50 

25.94 

21.40 

101.31 

489. 

H  18 . 

13.05 

17.71 

27.57 

26.58 

19.22 

104.16 

444. 

H  20 . 

1.92 

2.75 

42.01 

32.47 

23.97 

103.13 

553. 

H  21 . 

0.34 

0.80 

24.34 

42.77 

34.62 

102.89 

783. 

H  23 . 

1.80 

2.86 

29.95 

40.82 

27.30 

102.74 

634. 

G  2 . 

13.17 

2.47 

52.57 

20.57 

15.72 

104.52 

366. 

In  Table  IX  is  given  the  calculated  loss  on  ignition  of  each  group  as 
nearly  as  it  could  be  determined  from  the  results  of  analysis.  While 
this  loss  on  ignition  has  been  called  “Combined  Water,”  it  must  be  borne 
in  mind  that  the  loss  of  many  substances  other  than  combined  water 
has  been  included.  Carbon,  carbonic  acid,  sulphur,  etc.,  are  driven  off 
on  ignition  and  reduce  the  weight  of  the  sample.  The  relations  referred 
to  are  well  expressed  by  the  well-known  Kennedy  curves.  (See  Fig.  6.) 

Table  IX.  Distribution  of  combined  water  over  the  several  groups 

of  particles. 


Sample  Number. 

1  M.  M. 

1-.1M.  M. 

.1-.01 

.0— .001 

.001-0 

1 

Total. 

K  1 . 

0.32 

0.31 

1.62 

1.64 

1.86 

5.77 

K  2 . 

0.09 

0.08 

1.85 

1.37 

0.43 

3.84 

K  3 . 

0.06 

0.92 

2.28 

1.88 

1.86 

7.03 

K  4 . 

0.07 

0.05 

1.66 

1.25 

2.69 

5.74 

K  5 . 

0.31 

0.41 

1.98 

1.67 

1.64 

6.03 

K  6 . 

0.09 

0.08 

2.22 

2.33 

0.09 

4.82 

K  7 . . 

0.22 

0.31 

L69 

1.29 

2.00 

5.52 

K  8 . 

0.82 

0.30 

2.01 

2.69 

1.71 

7.55 

K  9 . 

0.27 

0.03 

1.18 

0.82 

1.02 

3  34 

K  10 . 

0.19 

0.28 

1.24 

2.83 

1.15 

5.71 

K  11 . 

2.27 

1.03 

2.58 

1.08 

1.61 

8.58 

K  12 . 

0.80 

0.63 

1.95 

2.49 

3.02 

8.91 

K  13 . 

0.20 

0.55 

1.68 

2.61 

1.68 

6.74 

K  14 . 

0.48 

0.33 

1.26 

1.54 

1.48 

5.11 

R  3 . 

0  66 

0.39 

1.72 

0.87 

1.75 

5.40 

R  4 . 

0.60 

0,27 

1.45 

2.13 

1.80 

6.27 

R  1 . 

0.39 

0.55 

1.74 

2.73 

2.43 

7.87 

I  -1 . 

0.40 

0.17 

0.24 

0.19 

3.51 

4.53 

G  -2  . 

0.33 

0.06 

1.13 

0.11 

2.75 

4.39 

H  18 . 

0.79 

5.45 

2.76 

5.49 

1.44 

15.95 

H-20 . 

0.89 

0.85 

1.52 

2.17 

6.95 

12.41 

H-23 . 

0.16 

0.23 

1.13 

1.67 

4.89 

8.11 

Fig/6.  Kennedy  curves  showing  the  reactive  rate  of  loss  on  heating  calcareous  and  non-caicereous  clays.  (After  Bleininger,  Geol.  Surv.  Ohio, 

4th  Ser.,  Bull.  4,  p.  19.) 


PURDY]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICKS.  153 


154 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  5> 


The  distribution  of  “combined  water”  over  the  several  groups,  as  given 
in  Table  I,  was  necessarily  calculated  by  proportion,  for  the  total  loss 
on  ignition  of  the  finer  groups  in  some  cases  amounted  to  two  or  three 
times  that  which  occurred  on  ignition  of  the  whole  sample.  Satisfactory 
explanation  of  this  increase  or  gain  in  volatile  matter  during  the  process 
of  analysis  cannot  be  given.  It  is  supposed,  however,  that  it  is  due  in 
part  to  some  organic  growth  developed  in  the  water,  or,  possibly,  oil  from 
the  compressed  air  that  was  used  in  the  siphoning  off  of  the  supernatant 
liquid.  That  this  last  suggestion  will  not  account  for  all  of  this  increase, 
if  any,  in  the  volatile  matter  accumulated  in  process  of  analysis,  was 
proved  by  the  fact  that  when  precautions  w£re  taken  to  clear  the  air 
of  all  possible  traces  of  solid  material,  there  was  still  nearly  the  same 
increase.  There  is  therefore  considerable  doubt  as  to  the  value  of  re-dis¬ 
tribution  of  the  loss  on  ignition  by  means  of  proportions,  yet  the  data 
obtained  in  this  way  are  considerably  more  accurate  than  they  would 
otherwise  be. 

The  irregularities  in  the  data  are  pointed  out  solely  to  call  attention  to 
a  weak  point  in  this  most  important  determination.  Mechanical  analysis 
of  clays,  as  has  been  stated  before,  bids  fair  to  become  a  very  essential 
test  in  determining  the  full  value  of  a  clay,  and  attention  should  be  given 
to  the  elimination  of  this  increase  in  volatile  matter  during  the  process 
of  analysis.  Soil  physicists  are  experiencing  the  same  difficulty,  and  yet 
they  have  learned  to  give  considerable,  in  fact,  a  large  amount  of  credit 
to  the  mechanical  analysis  of  soils  as  a  means  of  determining  its  proper¬ 
ties  for  their  purposes  . 


Shrinkage  in  Drying. 

METHODS  OF  MEASUREMENT. 

The  amount  that  a  clay  will  shrink  in  drying  is  expressed  in  per 
cents  of  the  unit  length  or  volume.  In  the  first  instance  the  shrinkage 
would  be  designated  as  linear  shrinkage,  and  in  the  second,  as  volume 
or  cubical  shrinkage. 

In  Table  I,  page  ?  ?,  will  be  found  the  percentages  of  both  linear  and 
volume  shrinkage  for  several  shale  clays  as  determined  by  direct  meas¬ 
urement.  It  will  be  noted  that  the  variation  in  linear  shrinkage  in  60 
bricks  of  each  clay  is  far  in  excess  of  reasonable  limits.  When  the  linear 
shrinkage  varies  from  32  to  133  per  cent  from  the  average,  the  data  must 
be  wholly  unreliable.  In  presenting  this  data  it  is  felt  that  the  failure 
to  produce  more  consistent  results  lies  in  part  in  the  shortness  of  the 
shrinkage  distance,  and  in  part  in  carelessness  of  the  operator.  In 
marking  the  freshly  made  bricks  a  stencil  devised  by  J.  F.  Ivrehbiel  was 
used,  so  that  initially  the  shrinkage  lines  were  marked  upon  the  brick 
with  accuracy.  In  measuring  the  decrease  in  length  of  the  shrinkage 
line  after  the  bricks  were  dried,  a  vernier  shrinkage  scale  was  used  that 
read  accurately  to  the  third  place.  The  large  variations  in  the  results 
were  therefore  a  surprise  to  the  operator. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICKS. 


155 


The  volume  shrinkage  varied  within  fairly  reasonable  limits,  but  even 
here  the  variations  are  quite  large  considering  the  size  of  the  bricks  used. 
It  is  felt  that  if  in  one  case  the  variation  could  be  only  0.5  per  cent  there 
ought  not  to  be  any  excuse  for  a  variation  of  33.8  per  cent  in  another 
or  an  average  on  all  samples  of  11  per  cent. 

Inasmuch  as  the  volume  shrinkage  data  proved  to  be  the  more  accur¬ 
ate  of  the  two  they  were  used  as  a  basis  on  which  to  calculate*  the  linear 
shrinkages  as  shown  in  the  following  table : 

Table  X.  Comparison  of  the  measured  with  calculated  linear 

shrinkage. 


Sample  No. 

Average 
volume 
shrinkage  in 
per  cents. 

Calculated 

linear 

shrinkage  in 
per  cents. 

Average 

measured 

linear 

shrinkage  in 
per  cents. 

Percentage 
variation 
on  volume 
shrinkage. 

Percentage 
variation 
in  measured 
linear 
shrinkage. 

K  1 . 

6.2 

2.1 

1.5 

33.8 

133.3 

K  2 . 

12.2 

4  3 

3.5 

2.4 

70.0 

K  3 . 

10.5 

3.6 

2  1 

16.7 

68.0 

K  4 . 

10.1 

3.5 

3.3 

5.8 

73.6 

K  5 . 

5.2 

1.8 

1.6 

6.7 

129.0 

K  6 . 

10.1 

3.5 

4.1 

10.3 

43.9 

K  7 . 

9.6 

3.3 

3.9 

12.2 

51.2 

K  8 . 

7.5 

2.6 

2.1 

21.5 

95.2 

K  9 . 

3.5 

1.2 

0.9 

34.1 

75.0 

K  10 . 

18.3 

6.5 

5.8 

0.5 

37.8 

K  11 . 

13.5 

4.7 

3.3 

16.3 

73.8 

K  12 . 

12.7 

4.4 

3.6 

5.9 

44.4 

K  13 . 

10.5 

3.6 

3.3 

7.5 

48.4 

K  14 . . 

6.1 

2.1 

1.5 

11.5 

93.3 

S  1 . 

12.9 

4.5 

2.7 

19.0 

60.0 

S  2 . 

13.1 

4.6 

4.2 

10.6 

42.8 

R  1 . 

13.9 

4.9 

4.5 

5.7 

53.3 

R  2 . 

9.1 

3.1 

3.3 

25.3 

36.3 

R  4 . 

6.1 

2.1 

3.2 

6.7 

56.2 

B  II . 

11.5 

4.0 

5.0 

7.8 

32.0 

C  II . 

7.3 

2.5 

1.9 

12.5 

124.0 

H  II . 

14.3 

5.0 

5.5 

7.6 

65.4 

I  II . 

13.8 

4.8 

4.2 

2.9 

33.3 

J  II . 

14.4 

5.1 

4.6 

4.1 

34.7 

L  II . 

9.7 

3.4 

3.7 

6.8 

54.0 

H  16 . 

7.8 

2.7 

2.8 

19.6 

78.5 

H  17 . . 

21.4 

7.7 

7.0 

4.6 

34.2 

H  20 . 

16.5 

5.8 

6.8 

3.0 

26.5 

H  21 . 

18.0 

6.4 

7.2 

1.7 

41.6 

H  23 . 

20.4 

7.3 

7.4 

5.8 

43.2 

H  24. . 

11.4 

4.0 

4.0 

10.5 

45.0 

*  If  a  unit  cube  shrinks  so  that  each  edge  is  decreased  by  linear  length  “a” 
then  the  new  length  of  the  edges  become  (1-a).  If  the  decrease  in  volume 
of  this  same  cube  be  represented  by  “x”  then  the  new  volume  will  be  (1-x). 
Since  the  edges  of  the  cube  are  now  (1-a)  its  volume  can  also  be  represented 

by  (1-a) 3  hence  (1-a) 3  is  equal  to  (1-x),  or  a=l-Vl-x.  It  was  by  this  form¬ 
ula  that  the  transformation  from  volume  to  linear  shrinkages  were  made. 


156 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


The  linear  shrinkage  which  probably  is  the  more  correct  for  that 
sample  is  underscored.  In  cases  where  there  is  not  an  underscored  linear 
shrinkage,  there  is  no  possible  way  to  judge  which  one  is  the  most  correct. 
In  case  the  calculated  practically  agrees  with  the  measured  linear  shrink¬ 
age,  both  are  underscored. 

If  the  volume  and  linear  shrinkages  had  been  correctly  measured,  there 
would  have  been  no  discrepancy  between  the  calculated  and  determined 
linear  data.  If  any  importance  at  all  is  to  be  attached  to  shrinkage  data 
it  is  evident  that  extreme  care  should  be  exercised  in  their  determination. 
When  possible,  the  measured  linear  should  be  checked  by  calculation 
from  the  volume  shrinkage  and  vice  versa. 

RELATION  OF  VOLUME  SHRINKAGE  TO  POROSITY. 

It  will  be  noted  from  a  glance  at  Fig.  7  that  there  does  not  seem  to  be 
any  relation  whatever  between  volume  shrinkage  and  pore  space  in  the 
dried  bricks. 

Fig.  8  also  represents  the  same  sort  of  irregular  relation  between  the 
volume  shrinkage  and  pore  space  in  the  dried  brick  made  from  the  Iowa 
loess  clays.1 

RELATION  OF  VOLUME  SHRINKAGE  TO  WATER  OF  PLASTICITY. 

The  chart,  (Fig.  9),  showing  the  relation  between  the  percentage  of 
water  of  plasticity  and  the  volume  shrinkage  from  the  green  to  the 
dried  condition,  proves  that  while  there  is  some  indication  of  a  reciprocal 
relation  between  these  two  factors,  this  relation  cannot  be  affirmed. 


o  H 17 

O  H23 

11210 

c 

Kioo 

U20 

Rio 

°  HI 

OK12 

°H-ll 

cKll 

Si 

c 

c-t 

"i 

V 

o 

0  52 

o  H24 

RJ( 

OK  3 

)  OL-il 

oB-II 

A' 40 
0 

H 16 

oK  13 

OK  6 

K7 

o  C-ll 

oKS 

oKli  < 
OK5 

>K1 

5K9 

PERCENT  POROSITY  OF  GREEN  BRICK 


Fig, 


7.  Diagram  showing  relations  between  volume  shrinkage  and  'porosity  of  dried  brick. 
(From  data  in  Table  I.) 


1  la.  Geol.  Surv.,  Vol.  XIV,  1904, p.  109  and  113. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICKS. 


157 


RELATION  OF  YOLUME  SHRINKAGE  TO  WATER  IN  EXCESS  OF  THAT  REQUIRED 

TO  FILL  THE  PORES. 

It  would  seem  that  if  the  volume  had  been  determined  at  regular  in¬ 
tervals  as  the  bricks  lost  their  mechanical  water  by  evaporation,  the  per 
centage  up  to  the  time  that  the  brick  reached  its  maximum  shrinkage, 


n? 

r>« 

om 

0 13 

19< 

i 

270 

170 

031 

no 

O  32 

02 

nlO 

07 

©3 

O  22 

Oi* 

012 

°26 

0 20 

4.0 

°15 

o?.i 

< 

> 30 

08 

018 

O  21 

029  ( 

>5 

< 

>1 

- 1 

' 25 

280 

in 

PERCENT  POROSITY 

Fig.  8.  Diagram  showing  relation  between  volume  shrinkage  and  porosity  of  loess  clays 
from  Iowa.  (After  Beyer  and  Williams.) 

would  stand  in  closer  relation  to  the  volume  shrinkage  than  does  the  total 
mechanical  water  and  volume  shrinkage.  It  is  not  known  what  value 
such  a  test  would  have,  but  it  would  probably  be  considerably  more  than 
is  the  determination  of  total  mechanical  water  alone. 

In  Table  XI  is  shown  the  percentage  by  weight  of  water  that  would 
be  required  to  fill  the  pores  of  bricks  made  from  Iowa  clays  and  that 
which  is  in  excess  of  the  “pore  water.”  These  clays  were  ground  until 
they  would  pass  through  a  40-mesh  sieve,1  then  wetted  with  water  and 
thoroughly  wedged.  Grinding  the  clay  until  it  would  pass  a  40-mesh 
sieve  would  reduce  the  size  of  the  larger  grains,  and  to  some  extent  break 
down  bunches  of  grains  by  force  that  would  not  have  been  affected  by 
the  water  used  in  wedging.  The  data  is  of  interest  on  this  account  in 
connection  with  the  problem  of  shrinkage. 


1  la.  Geol.  Surv..  Vol.XIV,1904,p.  76. 


158 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[bull.  no.  9 


TABLE  XI. 


Clay. 

Per  cent 
Water. 

Porosity. 

Sp.  Gr. 

Vol.  of 
clay 
per  100. 

Per  cent} 
by  wt.  of 
wraterin 
pores. 

Per  cent 
by  w7t.  of1 
clay. 

Excess 
water  for 
plas¬ 
ticity. 

Flint  Brick  Co.,  top 
stratum . 

22.5 

17.31 

2.41 

82.69 

7.99 

92.01 

14.51 

Flint  Brick  Co.,  middle. .. 

25.0 

23.00 

2.51 

77.00 

10.64 

89.36 

14.36 

Flint  Brick  Co.,  bottom. .. 

25.0 

30.04 

2.40  - 

69.96 

15.83 

84.87 

9.87 

Flint  Brick  Co.,  green 
brick . 

25.0 

23.20 

2.52 

76.80 

10.71 

86.29 

14.29 

Iowa  Brick  Co.,  top 
stratum.  . 

17.5 

17.43 

2.53 

82.57 

7.52 

92.48 

9.98 

Iowa  Brick  Co.,  second 
from  top . 

25.0 

.  25.73 

2.46 

74.27 

12.92  | 

87.08  1 

12.07 

Iowa  Brick  Co.,  third 
from  top . 

27  5 

29.57 

2,40 

70.43 

15.32 

84.68  | 

12.18 

Iowa  Brick  Co.,  fourth 
from  top . 

25.0 

17.96 

2.45 

82.04 

8.20 

91.80 

17.80 

Iowa  Brick  Co.,  fifth 
from  top . 

25.0 

26.04 

2.37 

73.96 

12.93 

87-07 

12.08 

Iowa  Brick  Co.,  bottom 
stratum . 

27.5 

28.69 

2.36 

71.31 

14.76 

85.24 

12.74 

Cap.  C.  Brick  &  P.  Co., 
top  stratum . .' . 

30.0 

30.80 

2.69 

69.20 

14.43 

85.57 

15.57 

Cap.  C.  Brick  &  P.  Co., 
second  from  top . 

22  5 

25.25 

2.48 

74.75 

11.44 

88.56 

11.06 

Cap.  C.  Brick  &  P.  Co., 
third  from  top . 

22.5 

17.00 

2.53 

83.00 

7.49 

92.51 

15.01 

Cap.  C.  Brick  &  P.  Co., 
fourth  from  top . 

22.5 

25.13 

2.45 

74.87 

12.04 

87.96 

10.46 

Jester  Clay  Bank . 

20.0 

20.35 

2.49 

79.65 

9.30 

90.70 

10.70 

Harris  Brick  Yard . 

22.5 

24.33 

2.56 

75.67 

11.15 

89.85 

11.35 

Dale  Brick  Co.,  top  loess.. 

22.5 

18.14 

2.44 

81.86 

8.32 

91.68 

14.18 

Dale  Brick  Co.,  bottom 
shale . 

22.5 

28.98 

2.48 

71.02 

14.13 

85.87 

8.37 

Corey  P.  B.  Co.,  red  burn¬ 
ing  clay . 

25.0 

30.10 

2.54 

69.90 

14.49 

85.51 

10.51 

Corey  P.  B.  Co.,  buff 
burning  clay . 

25  0 

28.10 

2.54 

71.90 

12.86 

87.14 

12.14 

Colesburg  . 

27.5 

28.36 

2.62 

71.64 

13.12 

86.88 

14.38 

Storm  Lake  B.  &  T.  Co  . .. 

25.0 

19.27 

2.42 

80.72 

8  97 

91.03 

16.03 

Besley  Brick  Yard,  top 
loess . 

22.5 

29.77 

2.34 

70.23 

15.34 

84.66 

7.16 

Besley  Brick  Yard,  middle 
loess . 

22.5 

25.30 

2.32 

74.70 

12.73 

87.27 

9.77 

Besley  Brick  Yard,  bot¬ 
tom  loess . 

22.5 

24.03 

2.40 

75.97 

11.64 

88.36 

10.86 

Getham  Bros.,  inland 
loess . 

25.0 

22.43 

2.41 

77.57 

10.71 

89.29 

14.29 

Cap.  City  B.  &  P.  Co., 
bottom  stratum . 

22.5 

24.59 

2.40 

75.41 

11.96 

88.04 

10.54 

Cap.  City  B.  &  P.  Co., 
green  brick . 

22.5 

21.83 

2.51 

88.17 

8.97 

91.03 

13.53 

Granite  B.  Co.,  top 
stratum . 

22.5 

23,06 

2.25 

76.94 

11.75 

88.25 

10.75 

Granite  B.  Co.,  lower 
stratum . 

22.5 

22.41 

2.42 

77.59 

.  10.51 

89.49 

11.99 

Clermont  B.  &  T.  Co . 

20.0 

22.66 

2.58 

77.34 

10.10 

89.90 

9.80 

Am.  B.  &  T.  Co . 

30.0 

26.71 

2.51 

73.29 

12.67 

87.33 

17.33 

In  making  Table  XI  Byers’  and  Williams  figures  for  porosity/5'  specific 
gravityf  and  water  of  plasticity];  were  taken,  and  the  data  calculated  as 
follows : 

If  porosity,  or  volume  of  pore  space,  is  29.77  per  cent  in  a  unit  volume 
there  would  be  0.2977  parts  by  volume  of  pore  space,  and  1.0000 — 0.2977 
or  0.7023  volumes  of  clay.  On  the  assumption  that  the  pore  space  is 
filled  with  water  and  the  specific  gravity  of  the  clay  is  2.34,  there  would  be 


*Ia.  Geol.  Surv.,  Vol.  XIV,  1904. 


1 


Ibid  83. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICKS. 


159 


0.2977X1-00=0.2977  parts  by  weight  of  water,  and 
0.7023X2*34=1.6434  parts  by  weight  of  clay,  or  ex¬ 
pressed  as  per  cents — 15.3  and  84.6  per  cent  respectively  of  water  and 
clay.  This  15.3  per  cent  of  water  then  is  the  amount  of  water  by  weight 
that  would  be  required  to  fill  the  pore  spaces  in  a  brick  that  would  weigh 
100  at  the  time  when  all  the  particles  have  become  fixed  or  arranged  in 
the  exact  position  that  they  will  maintain  during  the  remainder  of  the 
drying  period.  This,  it  is  assumed,  would  give  the  weight  of  water  that 
remains  in  the  pores  of  the  bricks  at  the  time  the  clay  has  reached  its 


0H17 

H230 

- 

H2H 

>  Kio  o 

o  H20 

If  170 

ORl 

Kl2 

o 

Kll 

o 

°  Ill 

0 

p  ^ 

O  H24 

o  R2 

K 

K 

13  O  S2  03 

4  °% 

L-U  K1 

11 

o 

KH 
O  O 

OK5 

oK8 

OKI 

o  Hie 

oK9 

12  U  16  18 

PERCENTAGE  WATER  OF  PLASTICITY 


22 


Pig.  9.  Diagram  showing  relation  between  amount  of  water  required  to  develope  plasticity 
and  volume  shrinkage.  (Data  from  Table  I.) 

maximum  air  shrinkage.  This  amount  of  water  subtracted  from  the 
amount  required  to  develop  plasticity  would  give,  if  the  foregoing  as¬ 
sumption  is  correct,  the  amount,  the  amount  of  water  required  to  lubri¬ 
cate  the  particles  sufficiently  to  cause  a  state  of  mobility  which  we  have 
learned  to  designate  as  plasticity. 

Fig.  10  shows  that  there  is  some  reciprocal  relation  between  the  amount 
of  water  in  excess  of  that  required  to  fill  the  pores  of  a  dried  brick,  (as 
given  in  Table  XI)  and  the  volume  shrinkage. 

In  Table  XII  are  shown  the  calculations  on  the  Illinois  clays,  designed 
to  bring  out  the  same  facts  given  in  Table  XI.  In  this  table,  however, 
the  amount  of  hygroscopic  water  is  given  in  each  case  so  that  it  can  be 
reckoned  in  as  part  of  the  mechanical  water,  if  so  desired.  It  must  be 
borne  in  mind,  however,  that  the  amount  of  water  calculated  as  being 
in  excess  of  that  required  for  filling  the  pores  does  not  in  any  way  include 
the  hygroscopic  water.  The  hygroscopic  water  is  not  added  in  with  the 


160 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


PERCENT  BY  WEIGHT  OF  WATER  IN  EXCESS  OF  THAT  REQUIRED  TO  FILL  POKES 


Fig.  10.  Diagram  showing  relation  of  volume  shrinkage  to  water  in  excess  of  that  required  to 

fill  the  pores  in  Iowa  clays. 

TABLE  XII. 


Sample 

N  umber. 

1 

Plasticity 

water. 

Porosity. 

Sp.  Gr. 
by  pyc¬ 
nometer. 

Per  cent 
by  weight 
of  water 
required  to 
fill  the 
pores. 

Excess 
water  re¬ 
quired  for 
plasticity. 

Hygro¬ 

scopic 

w'ater. 

Vol. 

shrinkage. 

K-  1 . 

14.9 

26.0 

2.67 

11.6 

3.3 

2.01 

6.2 

K-  2 . 

16.77 

25.7 

2.56 

11.9 

4.8 

1.62 

12.2 

K-  3 . 

16.82 

25.6 

2.69 

11.4 

5.4 

2.43 

10.45 

K-  4 . 

16.27 

27  8 

2  67 

12  6 

3.6 

10.12 

K-  5 . 

13.06 

25  4 

2.65 

11.3 

1.8 

0.923 

5.17 

K-  6 . 

17.03 

28.9 

2.66 

13.2 

3.8 

1.23 

10.6 

K-  7 . 

17.57 

27.9 

2.64 

12.7 

4.9 

1.93 

9  62 

K-'8  . 

14.4 

25.2 

2.69 

11.1 

3.3 

1.70 

7.51 

K-  9 . 

13.4 

26.1 

2.70 

11.5 

1.9 

0.79 

3.54 

K-10 . 

19.6 

25.4 

2.69 

11.2 

8  4 

2.31 

18.29 

K-12 . 

13.35 

18.3 

2.67 

7.7 

5.7 

5.09 

12.74 

K-13 . 

16.3 

28.3 

2.70 

12  7 

3.6 

2.16 

10.54 

K-14 . 

13.6 

24.5 

2.64 

10.9 

2.5 

0  79 

6.13 

S-  1 . 

17.2 

23.0 

2.64 

10.1 

7.1 

4.76 

11.97 

S-  2 . 

16.6 

26.4 

2.72 

11.6 

5.0 

2.42 

13.1 

R-  1 . 

13.4 

17.8 

2.73 

7.9 

5.5 

1  95 

13.9 

R-  2 . 

13.0 

24.0 

2.72 

10.4 

2.6 

1.53 

9.1 

R-  4 . 

13.2 

21.8 

2.72 

9.3 

3.9 

2.28 

5.98 

B-II . 

17.7 

26  9 

2.67 

12.1 

5.6 

1.67 

11.5 

G-II . 

11.8 

22.4 

2,70 

9.6 

2.2 

1.14 

7.32 

H-II . 

16.5 

20.7 

2.68 

8.8 

7.7 

3.07 

14.3 

I-II  . 

14  4 

18.9 

2.67 

8.0 

6.4 

2.85 

13.8 

J-II  . 

16.5 

24.2 

2.70 

10.5 

6.0 

2  70 

14.4 

L-Il . 

16.4 

24.5 

2.70 

10.7 

5.7 

3.05 

9.7 

H-16 . 

16.2 

27.8 

2.70 

12.4 

3.8 

1.74 

7.8 

H-17 . 

16.6 

19.0 

2.60 

8.2 

8.4 

3.7 

21.4 

H  -20 . 

18.3 

23.9 

2.72 

10.3 

8.0 

2.58 

16.5 

H-21 . 

18.0 

21.6 

2.72 

9.2 

7.8 

3.98 

18.0 

H-23 . 

21.4 

24.9 

2.63 

11.1 

10  3 

2.05 

20.4 

H-24 . 

12.8 

14.5 

2,66 

5.9 

6.9 

1.63 

11.4 

PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


161 


water  of  plasticity,  because  there  is  some  doubt  as  to  just  where  and  how 
the  clay  retains  that  water  on  drying.  It  is  supposed  to  be  held  either  in 
or  between  the  grains,  and  does  not  greatly  exceed  the  amount  (on  ac¬ 
count  of  the  natural  humidity  of  the  air)  that  the  powdered  clay  would 
retain  as  moisture. 

The  porosity  data  used  are  those  given  in  Table  I. 

In  the  above  table  there  is  the  same  indication  of  a  reciprocal  relation 
between  the  “excess  water”  and  volume  shrinkage  as  noted  in  the  case 
of  the  Iowa  clays  and  shown  in  Fig.  11.  We  have  here  then  the  promise 
of  a  means  of  obtaining  analytically  a  line  on  the  drying  behavior  of  a 
clay  other  than  volume  shrinkage  taken  alone. 


is 

to 

s* 


Fig.  11— Diagram  showing  relation  of  volume  shrinkage  to  water  in  excess  of  that  required  to 
fill  the  pores  in  Illinois  clays. 


o 


— 

KlO  603.6 

552.9 

O  H20 

H21  / 

7Q2J9  ' 

/ 

/ 

/ 

397.2 

OJT1 

/ 

fi-l/ 

/ 

"Rl 

O  S2 

33Vh 

4m°i-n 

/ 

/ 403. 4 

Ol K12 

K4 

n  O 

355.9 
K13  O 

220.6p6 

°Bdl 

S4M 

/ 

$24  / 

/ 

O  R2 

300.3 

o K7  / 

/ 

/ 

oLrll 

3 

366.4 

o  Gil 

254. 4^ 

/ 

/ 

275.3 

286.8 

o  K5 

/ 

/ 

/ 

/ 

’CrFl 

4 

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Z _ 

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- T 

5 - T4 

EXCESS  WA  TER  PLUS  HYGROSCOPIC  WATER 


—11  Gr 


162 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


RELATION  OF  VOLUME  SHRINKAGE  TO  FINENESS  OF  GRAIN. 

The  volume  shrinkage  of  a  clay  is  a  reliable  index  of  its  drying  be¬ 
havior  only  within  certain  limits.  Take  for  instance  K — 14  and  H — 17, 
which  lie  close  to  the  extremes  of  minimum  and  maximum  volume 
shrinkage;  both  require  considerable  care  in  drying.  Roughly  we  can 


Fig.  12.  Diagram  showing  relation  of  volume  shrinkage  lo  fineness  of  grain. 

say  that  clays  which  exhibit  an  average  shrinkage  will  dry  safely,  and 
that  if  the  ware  exhibits  either  a  high  or  low  volume  shrinkage  it  can 
be  assumed  to  be  likely  to  occasion  trouble  in  drying.  But  knowing  this 
general  fact,  how  can  the  drying  behavior  of  a  particular  clay  be  esti¬ 
mated  ? 

It  has  been  suggested  that  clays  which  have  a  fair  range  in  size  of 
grain,  i.  e.,  not  too  large  a  proportion  of  either  the  largest  or  smallest 
grains,  can  be  dried  with  greatest  safety.  This  we  proved  to  be  true  for 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


163 


the  clays  plotted  near  the  middle  of  a  diagonally  drawn  dotted  line  in 
Fig.  12  were  the  easiest  to  dry  and  those  at  the  extreme  ends  the  most 
difficult. 

It  was  demonstrated,  however,  that  while  there  may  possibly  be  a 
reciprocal  relation  between  porosity  and  fineness  of  grain  in  the  naturally 
soft  and  loose-grained  clays,  there  is  no  trace  of  such  a  reciprocal  rela¬ 
tion  in  the  harder  clays,  like  shales,  because  the  cement  which  holds  the 
grains  is  not  broken  by  the  methods  of  preparation  usually  employed. 
It  has  also  been  shown  that  there  is  no  reciprocal  or  proportional  rela¬ 
tion  between  the  porosity  of  the  dried  ware  and  the  volume  of  shrinkage. 
This  same  lack  of  proportional  relations  was  found  between  water  of 
plasticity  and  volume  shrinkage,  as  well  as  water  of  plasticity  and  por¬ 
osity.  The  only  factors  that  seem  to  exhibit  any  proportional  relation 
with  volume  shrinkage  are  “excess  water”  and  fineness  of  grain.  These 
factors  alone  are  not,  however,  sufficient  evidence  on  which  to  base  an 
answer  to  our  query. 


Tensile  Strength. 

METHODS  OF  TESTING. 

One  of  the  vital  factors  affecting  the  drying  behavior  of  clays  is  their 
cohesion.  Many  ways  have  been  devised  to  measure  this  cohesion,  but 
the  tensile  strength  test  seems  to  be  the  most  popular.  Determinations 
of  tensile  strength  as  usually  made  and  reported,  have  so  large  a  per¬ 
centage  of  variation  that  they  are  practically  worthless.  This  has  been 
justly  attributed  to  the  personal  factors  entering  into  the  preparation  of 
the  test  pieces.  It  is  indeed  surprising  how  variable  the  results  can  be 
even  when  the  operator  uses  all  the  care  possible  in  wedging  the  clay 
and  pressing  the  briquette.  The  personal  factors  have  been  largely 
eliminated  in  the  tests  here  reported  by  following  a  method  for  making 
briquettes  devised  by  H.  B.  Fox,  of  the  University  of  Illinois. 

Fox  Method. — The  Fox  method  is,  in  the  main,  as  follows :  The  clay 
is  mixed  with  just  sufficient  water  to  make  a  thick  paste.  It  is  allowed  to 
stand  in  this  condition  for  some  time,  generally  twelve  or  more  hours, 
and  is  then  poured  onto  a  slightly  moistened  plaster  slab  and  allowed  to 
harden  until  it  has  assumed  about  the  consistency  of  “stiff  mud.”  It  is 
then  cut  into  briquettes  by  a  cutter  similar  to  a  biscuit  cutter.  The  clay 
is  forced  out  of  this  cutter  into  the  briquette  mold  by  a  plunger  under 
a  given  load;  in  our  case  about  50  pounds.  While  the  load  is  still  on, 
the  cutter  is  removed  and  the  briquette  struck  off  with  a  wire.  By  this 
means  the  briquette  is  formed  and  pressed  under  uniform  conditions 
without  the  introduction  of  personal  factors, .  with  the  possible  exception 
of  the  making  up  of  the  slip. 

The  briquettes  are  then  room-dried.  In  this,  care  is  exercised,  for 
the  fine-grained  clays  and  the  exceptionally  weak  clays  can  be  dried  so 
fast  as  to  cause  them  to  “dry  check.”  It  is  not  always  possible  to  see 
these  “dry  checks,”  but  there  is  no  doubt  that  a  considerable  proportion 


164 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


Fia.  13.  Krehbiel  device  for  grooving  briquettes. 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  165 

The  briquettes  are  then  grooved  to  a  slight  depth  by  the  use  of  a  file 
operated  in  a  miter/  making  a  uniform  crpss  section  in  all  cases.  The 
object  of  this  grooving  is  not  to  obtain  a  uniform  cross  section  primarily, 
but  to  insure  the  breaking  of  the  briquette  at  the  narrowest  section.  Be¬ 
ing  uniform,  the  cross  section  can  be  considered  as  a  constant  factor,  thus 
making  easier  the  calculation  of  the  results.  This  grooving  was  not 
trusted  to  give  us  a  constant  cross  section,  however,  but  each  briquette 
was  measured  with  a  vernier  shrinkage  scale  that  reads  to  three  places. 

The  results  of  grooving  the  briquettes  may  be  noted  in  the  table  given 
below.  There  it  will  be  seen  that  the  strength  per  square  centimeter 
cross  section  is  not  materially  different.  In  fact  the  only  difference  in 
strength  between  the  grooved  and  the  ungrooved  can  be  said  to  be  within 
the  limits  of  errors  that  are  unavoidable  in  this  test.  The  usual  contrast 
between,  the  variation  in  results  in  the  grooved  and  in  the  ungrooved 
briquettes  which  ordinarily  exists  cannot  be  seen  in  the  result  given  be¬ 
low.  The  results  are  exceptionally  good  in  all  cases,  irregular  results 
due  to  breaking  elsewhere  than  at  the  neck  not  being  reported. 

After  the  briquettes  are  grooved  they  are  made  bone  dry  in  a  hot  air- 
bath  and  cooled  in  a  dessicator  so  as  to  eliminate  all  moisture,  and  then 
broken  in  a  Fairbanks  Tensil  Strength  Machine. 

Wedging  Versus  Slip  Process. — Clay  workers,  especially  the  old  potters 
who  make  large  jars  by  “throwing”  on  a  wheel,  recognize  a  difference  in 
the  working  properties  of  clay  when  prepared  by  the  slip  process  and 
when  prepared  by  the  “chaser,”  wet  pan,  or  the  old-time  stamping  pro¬ 
cess.  In  fact  the  difference  in  the  clay  when  prepared  in  slip,  or  in  one 
of  the  “plastic”  methods,  is  so  marked,  that  where  ware  is  to  be  thrown 
they  install  special  machinery  on  which  to  prepare  the  clay,  and  in  one 
of  the  most  up-to-date  terra-cotta  factories  in  the  west,  they  keep  four 
men  tramping  the  wet  clay  with  their  bare  feet,  in  preference  to  using 
the  cheaper  slip  method.  In  the  manufacture  of  glass  pots,  tramping 
with  bare  feet  is  the  method  most  generally  used  in  preparing  the  clay. 
For  this  reason  the  fairness  to  all  clays  in  casting  the  slab  from  which 
the  briquettes  were  cut  was  questioned,  and  the  following  tests  were 
made  to  throw  light  on  this  point. 

All  the  clays  for  both  the  “slip”  and  “wedge”  process  were  made  to 
pass  through  a  10-mesh  sieve. 

The  clay  for  slip  process  was  cast  as  in  the  Fox  method. 

The  clay  for  the  wedge  process  was  thoroughly  wedged  by  hand  while 
at  its  state  of  maximum  plasticity,  and  then  worked  into  a  sheet  1%" 
thick  on  the  plaster  slab  by  pounding  it  with  a  flat  board.  Briquettes 
were  cut  and  forced  into  the  mold  under  constant  pressure  as  in  the 
case  of  the  slip  clay. 

The  results  are  shown  in  Table  XIII : 


2  See  Fig.  13. 


166 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


TABLE  XIII. 


Slip  Process 

Per  Cent 
Variations 
Using  Aver¬ 
age 

Strength 
as  Basis. 

Wedge 

Process. 

Per  Cent 
Variations 
Using  Aver¬ 
age 

Strength 
as  Basis. 

Sample. 

average 

STRENGTH  IN 
LBS.  PER  SQ. 
CM. 

average 

STRENGTH  IN 
LBS.  PER  SQ. 
CM. 

Grooved. 

Not 

grooved. 

Grooved. 

Not 

grooved. 

Grooved . 

Not 

grooved. 

Grooved . 

Not 

grooved. 

K— 14  Western  Brick  Co.,  Dan¬ 
ville,  ill . 

16.20 

17.60 

9.9 

4.5 

28.9 

18.00 

6.9 

12.2 

K— 10  Terre  Haute,  Ind . 

29.80 

33.30 

10.3 

11.9 

36.8 

31.6 

17.1 

25.3 

K— 3  Albion,  Ill . 

17.65 

20.40 

10.3 

8.8 

23.9 

23.9 

16.7 

18.4 

K— 11  Brazil  shale,  Ind . 

17.85 

21.90 

13.3 

13.0 

24.0 

24.3 

37.5 

30.0 

K— 9  Crawfordsville,  Ind . 

8.25 

9.25 

8.5 

8.05 

9.3 

9.9 

3.35 

11.1 

K— 8  Veedersburg,  Ind . 

16.85 

18.60 

5.37 

4.0 

21.7 

22.1 

14.3 

21.2 

Average . 

17.60 

20.17 

9.61 

8.37 

24.1 

21.60 

15.97 

16.3 

The  following  conclusions  were  reached  as  a  result  of  these  tests : 

First — In  every  case  except  that  of  the  Terre  Haute  not  grooved,  the 
wedging  process  gave  higher  results. 

Second — The  variation  is  considerably  lower  in  the  slip  than  in  the 
wedge  process. 

Third — The  increased  strength  .due  to  wedging  was  not  sufficient  to 
warrant  the  accompanying  increase  in  percentage  of  variation. 

Fourth — Grooving  the  briquettes  did  not  materially  better  the  results 
in  the  slip  process  and  actually  made  the  results  worse  in  the  wedge 
process.  It  must  be  remembered  in  this  connection,  however,  that  the 
results  of  briquettes  that  did  not  break  at  the  necks  were  rejected.  All 
grooved  briquettes  broke  at  the  neck. 

Fifth — Grooving  increased  the  variation  in  coarse  non-plastic  clays, 
such  as  K — 14  and  K — 9,  but  did  not  seem  to  effect  the  finer  grained 
clays. 

Effect  of  fine  Grinding — In  view  of  the  fact  that  grooving  aids  ma¬ 
terially  in  reducing  the  variation  in  all,  except  the  less  plastic,  coarse 
grained  clays,  it  was  thought  that  perhaps  the  comparison  would  be 
more  just  if  all  were  finely  ground. 

The  dry-pan  samples  of  the  two  plastic  clays,  K — 10  and  K — 11, 
and  the  two  coarse  and  less  plastic  clays,  K — 14  and  K — 9,  were  ground 
of  the  variations  are  due  to  them. 

wet  and  also  dry  until  they  passed  through  sieves  of  10,  20,  40  and  80 
mesh,  as  follows : 

A  quantity  of  clay  sufficient  to  make  six  briquettes  was  taken  from 
the  stock  by  quartering,  making  ample  allowance  for  waste.  This 
sample  was  first  crushed  to  pass  a  10-mesh  sieve.  It  was  then  sieved 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BEICK. 


167 


through  the  desired  mesh  and  the  residue  placed  in  a  small  Bonnot  mill 
with  100  Iceland  pebbles.  Both  the  wet  and  the  dry  samples  were 
taken  from  the  mill  every  five  minutes,  and  the  particles  fine  enough 
to  pass  through  the  desired  mesh  were  sieved  out.  The  residue  left  on 
the  sieve  was  then  placed  in  the  mill  and  ground  for  another  five 
minutes.  This  grinding  was  continued  until  all  the  clay  passed  through 
the  desired  mesh.  In  this  manner  there  was  prepared,  by  both  wet  and 
dry  grinding,  stocks  that  would  just  pass  the  10,  20,  40  and  80  mesh 
sieves. 

The  clays  that  were  ground  were  kept  at  casting  consistency,  i.  e., 
quite  thick  slush,  so  that  when  completely  ground  they  were  cast  into 
slabs  as  quickly  as  convenient.  The  clays  prepared  by  the  dry  method 
were  allowed  to  stand  in  water  until  they  assumed  the  thick  slip  state 
and  then  cast  on  plaster  of  Paris  slabs  after  standing  from  10  to  24 
hours. 

Briquettes  were  cut  and  pressed  by  the  Fox  method.  In  table  XIY 
will  be  found  the  results  of  this  experiment. 

The  work  was  done  by  a  man  not  accustomed  to  it  who  could  not  at 
first  be  made  to  realize  the  importance  of  taking  .the  greatest  pains  to  in¬ 
sure  constant  conditions  and  accurate  results.  This  may  account  for  the 
higher  variations. 

From  these  results  the  following  conclusions  may  be  drawn: 

First — The  variations  with  the  grooved  briquettes  are  on  the  whole 
lower  than  those  with  the  ungrooved. 

Second — The  average  strength  of  the  grooved  is  practically  equal  to 
that  of  the  ungrooved. 

Second- — The  average  strength  of  the  grooved  is  practically  equal  to 
that  of  the  ungrooved. 

Third — Finer  grinding  either  wet  or  dry  does  not  materially  better 
the  constancy  of  the  results.  The  fact  is,  in  this  experiment,  the  varia¬ 
tions  in  the  finer  ground  samples  were  higher  in  many  cases  than  in 
the  coarsely  ground  samples. 

Fourth — The  average  strength  of  the  clay  was  not  materially  altered 
by  finer  grinding. 

Fifth — The  results  by  wet  grinding  differed  but  little,  if  any,  from 
those  by  dry  grinding. 


TABLE  XIV. 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS, 


[BULL.  NO.  9 


Wet  Grinding. 

Per  cent  variation 
ungrooved  . 

22.28 

17.97 

2.94 

27.33 

4.14 

25.29 

22  22 

15^92 

20.00 

15.83 

10.00 

17.62 

11.11 

13.5 

10.56 

10.8 

Per  cent  variation 
grooved  . 

21.16 

13.21 

27.82 

32.63 

9.89 

12  92 

25.00 

8.81 

11.42 

1.2  5 

13.25 

0.684 

6.29 

18.9 

21.79 

STRENGTH  IN  LBS.  PER 
SQ.  CM. 

Ungrooved. 

Min. 

26.5 

28.3 

33.5 

24.4 

19.89 

17.84 

19.85 

21.31 

6.0 

7.0 

8.1 

8.6 

14.3 

14.0 

11.0 

11.5 

Max. 

IO  OO  lO  CO 

▼HiOCOCD  c^oococo  wwoh  nncooj 
^  co  co  o’  co  in  t-oo’cio  coco  cm  cm* 

CO  CO  00  CO  CM  CM  CM  CM  HrtHH 

Grooved. 

Min. 

C^COCDlO  COlOlOlO  CM  -C~0O  ICOWCM 

lO^^CM  tr-  lO  C—  CM  CO  •  CO  tr- 

CM  CM  CM  CM  HHHCM  HHHH 

Max. 

NOONI>  O  •  t-  C—  CO  05  **  O 

cgoo^co  os  c-  co  c~  •  r-  oo  Tinotoin 

MNWM  HHNW  i-(  iH  t-4  iH 

Grinding  duration 
in  minimum . 

■OiOO  •  lO  o  o  •  o  o  o  •  o  o  o 

•Hr-ICM  •  rH  CM  CM  |H  HCM  -HHN 

Dry  Grinding. 

Per  cent  variation 
ungrooved  . 

14.99 

26.34 

7.42 

25.74 

21.16 

12.09 

22.27 

15.60 

15.11 

9.19 

9.78 

12.66 

7.26 

17.05 

18.00 

41.50 

Per  cent  variation 
grooved  . 

5.41 

32.59 

11.32 

22.5 

14.35 

7.94 

36.79 

11.15 

1.33 

4.93 

17.07 

17.7 

12.00 

5.76 

14.42 

12.42 

STRENGTH  IN  LBS.  PER 

SQ.  CM. 

Ungrooved. 

Min. 

CM  Oi  tCOOCOH 

Cl  Ci  CM  ^  HidCOO  CO  O  CO  o  coco^u- 

cdcocoir-  ocococo*  c-t-ooco  co^co*-* 

CO  CM  CO  CM  rH  CM  CM  CM  HHHH 

Max. 

39.08 

32.57 

39.10 

36.9 

27.29 

25.72 

30.40 

30.81 

8.6 

8.7 

9.2 

7.9 

17.9 
17.6 
20.0 
20.0 

Grooved. 

Min. 

31.7 

27.3 

31.4 

34.1 

17.3 

19.7 

14.6 

26.6 

7.4 

7.7 

6.8 
7.0 

15.4 

14.7 

14.8 

14.1 

* 

Bj 

S 

33.4 

40.5 

35.4 
44.0 

20.2 

21.4 

23.1 
26.9 

7.5 
8.1 
8.2 

8.5 

17.5 

15.6 
17.3 

16.1 

Grinding  duration 
in  minimum . 

•  moo  -moo  -moo  •  o  m  o 

■MCOO  tHNM  'NNM 

.  #  •  • 

.  .  •  • 

Mesh. 

oooo  oooo  oooo  oooo 

thSj^OO  SlM^OO  TH  OJ  ^  00 

Sample. 

K10  Teire  Haute . . 

Kll  Brazil . 

K9  [Crawfordsville . 

K14  Western  Brick  Co.,  Danville _ 

PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BEICK. 


169 


RESULTS  OF  TESTS. 

In  the  light  of  the  foregoing  tests  it  was  decided  to  dry-grind  the 
clays  in  a  jaw  crnsher  to  pass  a  20-mesh  sieve.  In  this  the  whole  sam¬ 
ple,  including  the  fine  and  coarse  particles,  was  passed  through  the 
jaw  crusher.  Six  briquettes  were  made  by  the  slip  method,  as  designed 
by  Fox,  and  grooved  to  insure  breakage  at  the  neck.  In  this  manner 
the  following  data  was  obtained: 

TABLE  XV. 

Tensile  Strength  of  Claves. 


Sample. 


K-  1  Alton,  Ill . 

K—  2  Hydraulic,  St.  Louis,  Mo . 

K-  3  Albion,  Ill . 

K—  4  Springfield,  Ill . 

K—  5  Edwardsville,  Ill . 

K-  6  Galesburg,  Ill . 

K—  7  Streator  Paving  Brick  Co . 

K—  8  Veedersburg,  Ind . 

K—  9  Crawfordsville,  Ind . 

K— 10  Terre  Haute,  Ind . 

K— 11  Brazil,  Ind  . 

K— 12  Brazil  Fire  Clay . 

K— 13  Clinton,  Ind . 

K— 14  Western  Brick  Co.,  Danville,  111 

K— 15  Barr  Clay  Co..  Streator,  Ill . 

H— 16  Carter,  Peoria . 

H— 18  Sterling,  Ill . 

H— 20  Savanna,  Ill . 

H— 21  Galena,  Ill . 

H— 23  Carbon  Cliff,  shale . 

H— 24  Carbon  Cliff  ‘  fire  clay . 

R—  1  Nelsonville,  Ohio . 

R—  2  Portsmouth,  Ohio . 

R—  3  Canton*  Ohio,  Imperial  plant _ 

R—  4  CantonOhio,  Royal  plant . 

S  —  1  Moberly,  Mo . 

S—  2  Kansas  City,  Mo . 

B— II  Atchison,  Kan . 

G— II  Coffeyville,  Kan  . 

H— II  Topeka,  Kan . 

I  —II  Caney,  Kan . 

J— II  Pittsburg,  Kan . 

L— II  Lawrence.  Kan . 

F—  1  Danville  Brick  Co . 


Strength  in  kilograms 
per  sq.  cm. 

Percent  of  Var¬ 
iation. 

Maxi¬ 

mum. 

Mini¬ 

mum. 

As 

tested. 

By  elimi¬ 
nation 
of 

irregular¬ 

ities. 

7.356 

7.168 

7.58 

12.292 

8.437 

31.52 

9.934 

8.074 

18.72 

6.84 

10.806 

8.664 

18.8 

11.1 

5.715 

5.216 

8.73 

8.164 

7.516 

7 . 202* 

6.985 

5.896 

15.62 

5.359 

4  717 

11.9 

4.373 

3.773 

13.7 

13.245 

11.762 

5.508 

9.525 

8.664 

9  03 

12.971 

12.201 

6.78 

8.346 

6.713 

19.5 

6.713 

5.359 

20.1 

14.9 

6.124 

5.629 

8.08 

6.033 

5.307 

12.03 

7.212 

8  942 

12  5 

8.210 

5.806 

29.2 

9.163 

7.666 

16.3 

23.406 

15.558 

37.8 

8.664 

7.503 

13.4 

11.114 

8.936 

19.6 

6.93 

9.662 

7.393 

23.4 

5.216 

4.717 

9.56 

5.359 

4.717 

7.97 

10.251 

7.892 

23.00 

10.513 

9.252 

11.9 

9  753 

8  664 

11.1 

9.254 

8.014 

13.4 

14.469 

13.880 

4.4. 

13.742 

12.383 

7.04 

10.069 

9.662 

8.09 

8.925 

8.028 

10.0 

12.111 

10.523 

13.1 

'  CAUSE  FOR  VARIATION  OF  MORE  THAN  15  PER  CENT. 

K-2.  There  were  two  briquettes  that  broke  with  high  strength  and  two 
with  low  strength. 

K-3.  There  was  one  briquette  that  broke  with  low  strength.  By  throw¬ 
ing  out  that  briquette  the  variation  would  be  reduced  to  6.84. 

K-4.  There  was  one  briquette  that  broke  with  high  strength.  By  throw¬ 
ing  out  that  briquette  the  variation  was  reduced  to  11.1. 

K-7.  There  were  two  briquettes  that  broke  with  low  strength. 

K-13.  There  were  two  briquettes  that  broke  with  low  strength. 

K-14.  There  was  one  briquette  that  broke  with  low  strength.  Elimination 
of  this  briquette  would  make  the  variation  14.9. 


170 


PAVING  BEICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  £ 


H-20.  There  was  one  briquette  that  broke  with  high  and  another  with 
low  strength. 

H123.  There  was  one  briquette  that  broke  with  high  and  another  wTith 
low  strength. 

R-l.  There  was  one  briquette  that  broke  with  low  strength.  Elimination 
of  this  briquette  would  make  the  percentage  only  6.93. 

R-2.  Three  of  these  briquettes  broke  with  high  and  three  with  low 
strength. 

S-l.  There  was  one  briquette  that  broke  with  high  and  one  with  low 
strength. 

The  results  here  reported  are  exceptionally  good.  The  variation  in 
the  strength  of  dry  clay,  as  made  by  other  methods,  usually  runs  from 
25  to  50  per  cent  in  nearly  every  reported  instance.  In  fact,  it  is 
seldom,  if  ever,  that  a  report  on  tensile  strength  will  show  a  lower 
variation  than  25  per  cent.  The  placing  of  15  per  cent  as  the  maxi¬ 
mum  variation  to  be  allowed  would  be  very  severe  standard  ordinarily, 
but  the  general  character  of  the  work  as  here  reported  justifies  the  limit. 

RELATION  OF  TENSILE  STRENGTH  TO  FINENESS  OF  GRAIN. 

Curves  were  plotted  from  data  given  by  Bies1,  and  also  by  Beyer  and 
Williams2,  showing  the  relation  between  fineness  of  grain,  as  delineated 
by  the  surface  factor,  and  tensile  strength.  There  did  not  appear  to  be 
any  consistent  relation  between  these  two  factors,  shown  by  the  curves. 


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yka 

TENSILE  STRENGTH  IN  KG.  PER  SQ.  CM 
Fig.  14.  Diagram  showing  relation  between  tensile  strength  and  fineness  of  grain. 

Notwithstanding  the  apparent  contradiction  in  the  case  of  the  New 
Jersey  and  Iowa  clays,  it  is  believed  that  fineness  of  grain  in  a  given 
clay  does  bear  a  relation  to  the  tensile  strength.  Orton1  has  shown  the 
influence  of  different  sized  grains  upon  a  very  close  grained  and  tough 

1  New  Jersey  Geological  Survey,  Vol.  6,  p.  89. 

2  Iowa  Geological  Survey,  Vol.  XtV,  p.  84. 

1  Trans. Amer.Cer.Soc.,  Vol. Ill, p. 117. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAVING  BRICK. 


171 


ball  clay.  This  clay  is  so  fine  by  itself  that  it  is  extremely  difficult  to 
dry  without  air  checking,  but  with  increasing  adulteration  of  sa$d  up 
to  30  per  cent  by  weight,  the  tensile  strength  increased  up  to  a  maxi¬ 
mum  in  the  sample  where  the  sand  was  of  extreme  fineness,  and  here 
again  the  tensile  strength  decreased  rapidly.  This  drop  in  the  curve  is 
credited  to  the  inability  of  the  extremely  fine  mixture  to  part  with  its 
mechanical  water  without  checking,  thus  causing  flaws  in  the  briquette 
and  very  materially  weakening  it.  In  this  experiment  we  have  at 
both  extremes  very  fine  grained  materials;  one  a  pure  ball  clay  and  the 
other  the  same  ball  clay  adulterated  by  fifty  per  cent  by  weight  of  a 
very  fine  sand,  both  having  a  low  tensile  strength.  The  intermediate 
members  of  this  series  show  increasing  strength  with  decrease  of  size 
of  grain.  So  far  at  least  as  this  one  case  is  concerned,  increase  in  size 
of  grain  increases  tensile  strength.  Fineness  of  grain  and  tensile 
strength  are,  therefore,  functions  of  one  another. 

We  know  that  a  fine-grained  shale  is,  in  a  majority  of  cases,  im¬ 
proved  by  adulteration  with  sandstone,  even  in  the  fact  of  the  fact 
that  the  sandstone  is  very  co.arse.  At  Streator,  Ill:,  there  are  two  strata 
of  shale  in  one  bank,  the  one,  being  very  gritty,  is  easily  manufactured 
into  a  good  paver;  the  other,  a  close  grained  plastic  shale,  gives  trouble 
in  every  stage  of  manufacture,  and  makes  a  poor  paver.  Yet  these 
two  shales  are  said  to  be  of  very  similar  chemical  composition.  The 
writer  believes  that  the  cause  of  this  difference  does  not  lie  in  their 
chemical  composition,  shrinkage,  or  ability  to  slake  easily,  but  in  their 
drying  behavior.  Judging  from  the  results  of  Prof.  Orton’s  experi¬ 
ment  on  the  tough  ball  clay,  it  is  believed  that  if  many  of  the  plastic, 
fine  grained  clays  were  by  addition  of  coarse  material  opened  suffi¬ 
ciently  to  permit  ready  egress  of  the  mechanical  water,  they  would  be 
excellent  paving  brick  material1,  while  without  such  a  treatment  they 
would  be  worthless  for  anything  other  than  building  brick,  simply  be¬ 
cause  the  bond  of  the*clay  would  be  weakened  in  drying  by  the  expand¬ 
ing  steam  inside  of  the  brick  which  could  not  readily  escape. 

In  Fig.  14  data  are  plotted  showing  the  relation  between  fineness  of 
grain  and  tensile  strength.  This  is  indicated  by  the  dotted  line. 

It  will  be  noted  that  there  is  a  general  relation  between  fineness  of 
grain  and  tensile  strength.  This  is  indicated  by  the  dotted  line. 

There  is  a  remarkable  coincidence  in  the  relative  positions  of  the 
several  clays  in  Fig.  14  and  Fig.  11.  The  same  relative  positions  of  the 
several  clays  is  to  be  seen  also  in  Fig.  12  which  shows  the  relation  be¬ 
tween  volume  shrinkage  and  surface  factor.  This  same  relative  posi¬ 
tion  of  the  clays,  one  with  another,  was  developed  also  when  the  rela¬ 
tion  between  the  sum  of  the  excess  and  hygroscopic  water  and  the  surface 
factor,  and  also  the  relation  between  the  sum  of  the.  excess  and  hygro¬ 
scopic  water  and  the  tensile  strength,  were  plotted.  In  the  last  two  in¬ 
stances,  however,  the  order  in  which  the  clays  occurred  was  the  reverse 
of  that  in  Fig.  14  and  11. 

1  It  is  not  desired  that  the  reader  should  infer  that  this  is  suggested  as  a  panacea  for  all 
clays  or  that  all  clays  can  be  “doctored”  so  as  even  theroetically  to  make  them  fit  tor  paving 
brick  manufacture. 


172 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


RELATION  OF  TENSILE  STRENGTH  TO  VOLUME  SHRINKAGE. 

We  have  seen  that  there  is  a  greater  shrinkage  of  the  mass  when  dried 
from  stiff  mnd  to  bone  dryness,  as  the  grains  of  the  clay  decrease  in 
size.  If  these  fine  particles  are  composed  largely  of  clay  substance  they 
will  possess  a  degree  of  cohesion  that  will  cause  the  dried  mass  to  become 
quite  hard,  the  hardness  increasing  directly  with  increase  of  cohesion 
possessed  by  the  individual  particles.  With  increase  of  exposed  surface. 


$ 


Fig.  15.  Diagram  showing  relation  between  volume  shrinkage  and  tensile  strength. 

ought  to  be  an  increase  in  tensile  strength,  for  the  closer  the  particles 
are  to  one  another  the  greater  will  be  the  bond  between  them.  De¬ 
crease  in  size  of  grain,  increase  in  volume  shrinkage  and  increase  in  ten¬ 
sile  strength  should,  therefore,  follow  one  another  in  this  order  as 


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TENSILE  STRENGTH  IN  KC  PER  SQ.  CM. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


173 


causes  and  effects.  Such  a  relation  is  shown  in  Fig.  15  where  the  vol¬ 
ume  shrinkage  and  tensile  strength  are  plotted  coordinately. 

The  drying  behavier  of  a  given  clay  then  can  be  said  to  be  a  function 
of  four  factors  acting  simultaneously,  viz:  Volume  shrinkage,  excess 
water,  fineness  of  grain  and  tensile  strength.  The  greater  the  volume 
shrinkage  and  the  larger  amount  of  excess  water  present,  the  more  dan¬ 
ger  will  there  be  in  drying.  The  greater  the  fineness  of  grain  and  the 
larger  the  tensile  strength,  the  safer  ought  the  clay  to  dry,  all  other 
things  being  equal.  If  a  drying  modulus  were  to  be  formulated  it  would 
have  in  the  numerator  the  surface  factor  (S),  representing  the  fineness 
of  grain,  and  tensile  strength  (T)  ;  in  the  denominator  there  would  be 
the  percentage  of  volume  shrinkage  (1)  and  excess  water  (E),  that  is, 
S  T 

-  This  simple  relation  is  not,  however,  expressive  of  the  true  rela- 

V  E. 

tive  value  of  the  involved  factors.  It  is  believed  that  the  formula. 

-Ss  T 

V3  E  approximates  th^  truth  more  closely. 

100 


Plasticity. 

THEORIES  OF  PLASTICITY. 

There  is  probably  no  property  of  unburned  clay  which  has  been  more 
widely  discussed  than  plasticity.  To  plasticity  the  clay  owes  its  re¬ 
sponsiveness  to  every  touch  of  the  potter’s  hand  and  its  adaptability 
to  the  preservation  of  every  line  of  the  artist’s  tool;  it  is  this  quality  that 
permits  of  its  being  drawn  out  into  sheets  and  cylinders  of  the  most 
astonishing  thinness. 

Of  the  many  theories  advanced  as  to  the  cause  of  plasticity  the  fol¬ 
lowing  are  the  most  tenable: 

Molecular  Attraction  Theory — To  properly  appreciate  this  conception 
of  the  cause  for  plasticity,  suppose  clay,  to  be  blunged  into  the  form  of 
a  slip,  as  is  the  practice  of  the  potter  before  casting  a  vase.  In  this 
slip  or  fluid  condition  each  grain  is  surrounded  or  enveloped  by  a  film 
of  water.  If  the  volume  of  water  is  large  compared  with  the  total  vol¬ 
ume  of  clay  particles,  the  mass  will  behave  in  every  respect  like  a 
fluid;  indeed,  as  will  the  turbid  water  of  the  Mississippi.  Suppose  that, 
by  evaporation,  or  adsorption  by  a  plaster  mold,  the  volume  of  the 
water  be  decreased.  The  clay  particles  will  be  brought  closer  and  closer 
to  one  another,  causing  the  mass  to  pass  from  a  fluid  state  through  var¬ 
ious  stages  of  consistency  until  it  assumes  a  stiff  plastic  condition;  a 
process  to  be  observed  in  mud  roads  after  every  rain.  When  in  this 
stiff  condition  the  particles  still  have  an  envelope  of  water  or,  in  other 


174 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


words,  they  are  still  suspended  in  water  just  as  truly  as  they  were  when 
the  mass  was  more  of  a  fluid.  But,  owing  to  their  proximity,  it  is 
assumed  by  those  advancing  this  theory  of  plasticity,  that  they  are  held 
in  position  by  the  molecular  attraction  which  each  particle  of  clay  sub¬ 
stance  exerts  on  the  other. 

Molecular  attraction  is  a  known  force,  and  there  has  been  no  adequate 
proof  advanced  upon  which  positive  claims  can  be  made  against  such  a 
force  operating  between  clay  particles  when  brought  into  close  proxi¬ 
mity.  The  popular  conception  of  a  bar  of  iron  is  that  it  is  a  rigid 
homogeneous  mass,  but,  as  is  shown  in  magnetization  experiments,  it 
is  made  up  of  individual  particles  which  can  be  turned  about  or  set  up 
endwise,  thus  acting  independently  of  one  another  except  in  the  matter 
of  the  molecular  attraction  that  each  exerts  upon  its  neighbor,  binding 
or  holding  the  whole  together.  Aside  from  composition  the  degree  of 
molecular  attraction  determines  the  hardness  of  the  iron.  Iron,  then, 
is  a  solid  fluid,  that  is,  it  will  flow.  The  force  of  gravity  is  not  sufficient 
to  overcome  this  molecular  attraction  and  cause  flowage,  but  when  a 
force  that  exceeds  that  of  the  molecular  attraction  is  applied,  flowage 
follows  in  the  direction  of  the  greater  force.  It  is  in  this  respect  that 
iron  is  a  fluid. 

If  similar  flowage  is  attempted  when  the  grains  in  a  clay  mass  are 
practically  dry,  or,  in  other  words,  not  surrounded  by  water,  except  per¬ 
haps  that  held  by  absorption,  pressure  sufficient  to  overcome  the  force 
binding  or  holding  the  particles  together  will  disrupt  the  ware.  That  is, 
instead  of  flowage  of  the  particles  in  this  comparatively  dry  state,  rup¬ 
ture  is  a  possibility.  Further,  maximum  plasticity  or  ability  to  flow  i§ 
not  attained  until  the  maximum  number  of  particles  is  enveloped  with 
the  least  amount  of  the  suspending  medium.  This  same  phenomenon  is 
to  be  noted  with  almost  all  fine  insoluble  powders.  Wheeler1  has  shown, 
for  instance,  that  the  non-plastic  slates,  Iceland  spar,  propyllite,  gypsum 
and  halloysite  can  be  made  to  develope  a  much  smaller  but  still  a  fair 
degree  of  apparent  plasticity  with  water  as  a  floating  medium.  When 
dried,  the  force  required  to  disrupt  these  masses,  while  small,  is  yet 
comparatively  great.  The  difference,  however,  between  the  behavior  of 
clay  and  these  finely  pulverized  minerals  is  that  the  latter  can  be  molded 
by  pressure  alone  into  a  shape-  that  will  have  a  comparatively  higher 
tensile  strength  than  if  they  were  caused  to  acquire  that  shape  by 
flowage  due  only  to  assumed  plasticity.  But  we  know  that  maximum 
density  and  consequent  strength  can  be  best  developed  in  plastic  clay 
by  the  combined  influence  of  pressure  and  plasticity.  Now  is  it  mole¬ 
cular  attraction  in  the  case  of  clay,  as  in  that  of  iron  which  can  be  bent, 
stretched,  rolled,  etc.,  in  the  cold  without  rupture,  or  is  it  merely  that 
clay  grains  may  be  pressed  so  close  together  that  fiowTage  is  permitted 
so  long  as  water  is  present  in  excess,  but  is  resisted  by  fractional  force 
when  dry? 


^o.  Geol.  Surv.,Vol.  XI.  p.  106. 
2Carhart,  H.S., Univ. Physics, Pt.  I. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK. 


175 


Text  books  on  physics  give  as  an  “expression”2  for  the  force  of  mole¬ 
cular  attraction  between  two  molecules,  M  and  M',  MM'f  (r).  “All  that 
is  known  about  this  funtion  of  r  is  that  it  is  very  large  for  insensible 
distances,  that  it  diminishes  very  rapidly  as  r  increases  and  that  it 
vanishes  while  r  is  still  very  small.  The  maximum  value  of  r  at  which 
molecular  action  ceases  is  estimated  by  Quincke  to  the  0.00005  mm.;# 
If  the  particles  then  were  0.00005  mm.  or  0.00002  inches  apart,  they 
would  be  at  the  extreme  distance  through  which  molecular  attraction 
can  possibly  operate.  Grout1  says,  however,  “Now  a  simple  calculation, 
based  on  the  mechanical  analysis  of  the  clays,  will  show  that  the  amount 
of  water  needed  to  place  a  film  0.00005  mm.  thick  around  each  grain 
is  often  nearly  equal  to  the  amount  added  in  tempering,  so  that  in 
ordinary  plastic  clay,  it  is  necessary  to  consider  practically  all  the  water 
as  being  under  this  influence.” 

Grout2  bases  his  reasoning  on  the  following  calculations :  He  found 
that  his  “mechanical  analyses  frequently  show  a  large  percentage  of 
grains  below  0.001  mm.  in  diameter,  also  from  0.001  to  0.005  mm. 
The  average  diameter  of  grains  below  0.001  mm.  is  0.0005  mm.  If 
these  are  considered  spherical  and  of  specific  gravity  2.5,  it  would  re¬ 
quire  25.5  per  cent  by  weight  of  water  to  place  around  each  grain  a 
film  0.00005  mm.  thick.” 

On  making  these  same  calculations  the  following  was  obtained: 

PiD3 

Vol.  of  sphere^ - 

6 

Given  diameter  of  sphere  0.00005 

Pi  — 

Log  —  =  1.71899 

6  s  - 

Log  0.0005  =  10.09691 

11.81590  =  Log  6545  X  10-14  volume  of  clay- 

sphere. 

Diameter  of  sphere  plus  water  film  =  .0005  -+- 
.0001  or  .0006 
Pi  — 

Log  —  =3  1.71899 

6 


3  10.33445 

Log  .0006  = - 1 — 

10.05344  =  Log  1131  X{  10-is  vol¬ 
ume  in  cu.  mm.  of 
sphere  of  clay  plus 
water. 


Reducing  these  figures  for  the  sake  of  convenience  to 

0.6545  =  volume  of  clay  sphere, 

1.131  =  volume  of  clay  plus  water  sphere. 
0.6545  -^-  1.131  =  0.5775,  part  of  unit  volume  of 
clay  plus  water  sphere 
occupied  by  the  clay. 

1.00  —  .5776  =  0.4224,  part  occupied  by  water 

film. 


1  Jour.  Am.  Chem.Soc.,  Vol.  XXVII,  No. 9,  Sept.  1905. 

2  Loc.cit. , p.1046. 


176 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


Given  specific  gravity  of  clay  =  2.5 

Since  in  the  metric  system  Vol.  X  Sp.  Gr.=Weight 

0.5775  X  2.5=1.4540  parts  by  weight  of  clay 
0.4224  X  1.0=  .4224  parts  by  weight  of  water 


1.8764  total  weight. 

0.42246  -P  1.8764  =  0.2251,  parts  by  weight  of 

water  in  a  unit  vol¬ 
ume  of  clay  plus 
water  film,  or  22.5 
per  cent. 

This  calculation,  so  far  as  the  validity  of  Grout’s  argument  is  con¬ 
cerned,  checks  his  results. 

Grout  further  calculated  that  if  this  same  volume  of  clay  were  con¬ 
sidered  as  a  square  plate  one-fifth  as  thick  as  wide,  instead  of  a  sphere, 
over  54  per  cent  of  water  would  be  held  to  the  clay  particles  by  this1 
molecular  attraction.  Supposing  it  to  be  fair,  inasmuch  as  the  kaolin- 
ite  crystal  is  “plate-like,”'  to  consider  that  in  a  clay  half  of  the  grains 
are  approximately  spherical  and  the  remainder  plate-like.  Grout  fig¬ 
ures  that  a  clay  having  all  its  particles  the  size  here  assumed  would  take 
by  virtue  of  the  molecular  attraction  of  the  clay  particles,  40  per  cent 
of  water. 

In  a  personal  interview  the  writer  suggested  to  him  that  he  was 
taking  the  maximum  limit  of  the  distance  through  which  this  molecular 
attraction  can  be  said  to  operate.  His  defense  was  that  when  the 
spheres  were  devoid  of  a  water  film  they  touched  one  another,  but  as 
they  gathered  to  themselves  this  water  film,  they  need  not  necessarily 
be  separated  .00005  mm.,  for  the  film  crowded  from  the  points  of  closest 
proximity  could  be  considered  as  filling  up  the  space  that  would  other¬ 
wise  have  to  be  considered  as  void. 

It  must  be  admitted  by  the  supporters  of  Grout’s  molecular  attraction 
theory  for  plasticity,  that  he  used  but  a  portion  of  a  very  fine-grained 
clay  on  which  to  calculate  his  demonstrating  example.  If  he  had  taken 
into  consideration  the  data  for  the  sample  of  clay  as  published  by  him 
instead  of  only  those  for  the  finer  portions,  quite  different  results  would 
have  been  obtained  as  is  shown  in  Table  XYI. 

The  calculations  by  which  the  data  in  the  following  table  were  ob¬ 
tained  are, 

(a)  Volume  of  clay  sphere  PiD3  where  D  is  the  mean  diameter  of  the 

- -  range  in  each  group  of  the  mechanical 

6  analysis. 

Pi  (D  — 0.0001)3  — PiD3 

(b)  Volume  and  weight  of  water  film  -  - 

6  6 

(c)  Weight  of  dry  clay  particles  as  given  in  the  mechanical  analysis. 

id)  Total  or  collective  volume  of  spheres  in  each  group:  Weight  given 

-f-  Sp.  Gr.  of  the  clay. 

(e)  Number  of  spheres  in  each  group  per  unit  volume:  Total  volume  of 

d 

each  group  -p  volume  of  clay  sphere  or  — 

a 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


177 


( f )  Weight  of  water  film  surrounding  the  sphere  in  each  group  of  the 
sample:  Weight  of  water  film  times  the  number  of  spheres  or  e  X  b. 

( g )  Sum  of  water  required  to  give  each  particle  in  the  sample  a  water 
film  of  prescribed  thickness. 

TABLE  XVI. 


Sample. 

Sp.  Gr. 

Total  dry 
weight  of 
clav  par¬ 

Total 
weight  of 
water  films 

Analyst. 

Reference  to  data 
used  and  explanatory 

ticles  by 
analysis. 

by 

calculation 

notes. 

S.  C.  Besley,  top  clay  (1) 

2.34 

0.9844 

S.  C.  Besley,  middle  clay 

2.32 

0.9834 

S.  C.  Besley,  bottom  clay 

2.40 

0.9819 

Dale  Brick  Co . 

2.44 

0.9857 

Gethmann  Bros . 

2.41 

0.9739 

Clarksburg  fire  clay  (2)... 

2.52  . 

0.9880 

Bridgeport  stoneware 
clay  (2) . 

2.35 

0.9870 

Charleston  river  clay  (3) . 

2.66 

0.987 

Parkersburg  pottery  clay 

2.58 

0.984 

K-l  Alton,  Ill.  (4) . 

2.66 

1.045 

K-2  St.  Louis  Mo . 

2.56 

1.021 

K-3  Albion,  Ill . 

2.686 

1.002 

K-4  Springfield,  Ill . 

2.67 

1.037 

K-5  Edwardsville,  Ill... 

2.65 

1.028 

K-6  Galesburg,  Ill . 

2.66 

1.027 

K-7  Streator.  B.  B.  Co. . 

2.636 

1.037 

K-8  Veedersburg,  Ind. 

2.689 

1.005 

K-9  Crawfordsville.Ind. 

2.702 

1.008 

K-10  Terre  Haute, Ind... 

2.69 

0.979 

K-ll  Brazil  shale . 

2.659 

1.013 

K-12  Brazil  fire  clay . 

2.669 

1.021 

K-13  Clinton,  Ind . 

2.71 

1.048 

K-U  Western  P.  B.  C., 
Danville . 

2.72 

0.9899 

R-l  Nelsonville,  O . 

2.73 

1.023 

R-3  Canton,  O.  (Imper¬ 
ial)  . 

2.66 

1.043 

R-4  Canton, O. (Royal). 

2.72 

1.023 

—12  G 


0.0165 

V/  iams. 

pp.  116  and  123,  la.  Geol. 
Surv.,  Vol.  XIV. 

0.0219 

.  .do . 

Williams’  first  group  was 
termed“aboveO,l  MM.” 

0.0263 

..do . 

In  this  group  the  mean 
diameter  of  the  particles 
was  assumed  to  be  O. 
175  M.  M. 

0.0320 

.  .do . 

0.00254 

.  .do . 

0.0393 

Grout . 

pp.65and  251  W.Va.  Geol. 
Surv.,  Vol.  III. 

0.1750 

.  .do . 

pp.  65  and  162  W.Va.  Geol. 
Surv.,  Vol.  III.  Attract¬ 
ed  water=  15  -(-  per  cent. 

0.1009 

.  .do . 

pp. 65  and  200 W.Va.  Geol. 
Sury.,  Vol.  III. 

0.1043 

.  .do . 

pp. 65  and  160 W.Va.  Geol. 
Surv.,  Vol.  III. 

0.0356 

Krehbiel 
and  Moore 

0.0491 

0.0475 

0.0703 

0.0390 

0.0320 

0.045 

Krehbiel 
and  Moore 

0.036 

0.029 

Calculated 
by  Merry. . 

0.0804 

0.0467 

0.0533 

No.  2.  fire  clay. 

0.0479 

0.0348 

0.0519 

No.  2  fire  Clay. 

0.041 

0.03902 

178 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


Table  XVI — Concluded. 


Sample. 

Sp.  Gr. 

Total  dry 
weight  of 
clay  partic¬ 
les  by 
analysis. 

Total 
weight  of 
water  films 
by 

calculation 

Analyst. 

Reference  to  data  used 
and  explanatory  notes. 

I-ll  Caney,  Kan . 

2.67 

2.71 

2.67 

2.72 

2.72 

2.6,3 

1.013 

1.045 

1.1)42 

1.031 

1.029 

1.027 

0.0670 

0.0463 

0.0593 

0.0739 

0.10301 

0.0867 

G-ll  Coffeyville,  Kan... 

H-18  Sterling,  Ill . 

H-20  Savanna,  Ill  . 

H-21  Galena,  111 . 

H-23  Carbon  Cliff,  Ill. 
(shale)  . . 

(1)  The  Iowa  clays  are  loess. 

(2)  Stoneware  or  No.  2  fire  clays. 

(3)  Alluvial  clay. 

(4)  The  Illinois  clays  are  shales  in  every  instance  except  K-12  and  R-l. 

In  Table  XVI  there  is  but  one  instance  that  of  the  West  Virginia 
stoneware  clay,  in  which  the  amount  of  water  molecularly  attracted  even 
approached  that  required  to  develop  plasticity.  In  many  instances  it 
does  not  greatly  exceed  the  hygroscopic  water  that  the  clay  would  re¬ 
tain  when  dried  in  open  rack  dryers.  In  fact  the  maximum  amount 
of  water  which  Grout  admits  could  be  so  molecularly  attracted,  agrees 
quite  closely  with  the  water  which  in  Table  XI  is  shown  to  be  in  excess 
of  that  required  to  fill  the  pores.  While  Grout’s  statement  of  the  facts 
in  this  case  has  been  proved  incorrect,  further  investigation  may  find 
a  relation  between  the  molecularly  attracted  water  and  “excess  water.” 
As  yet,  however,  such  a  relation  cannot  be  established. 

That  a  clay  particle  does  possess  a  molecular  attraction  peculiar  to 
itself  is  not  denied.  That  this  molecular  attraction  alone  is  sufficient 
to  cause  a  plasticity  that  is  peculiar  and  belongs  to  no  other  substance 
must  be  discredited  until  evidence  is  brought  forward  that  will  bear 
an  analysis  such  as  is  given  in  Table  XIV. 

It  would  be  most  difficult  for  supporters  of  the  molecular  attraction 
theory  to  prove  that  the  kaolin  grains  in  primary  clays  do  not  possess 
every  physical  property  that  is  attributed  to  the  grains  of  the  clay  sub¬ 
stance  in  the  secondary  clays,  save  that  of  plasticity.  Chemically  alike, 
and  differing  physically  only  in  this  one  respect,  yet  to  the  one,  accord¬ 
ing  to  this  theory,  must  be  accredited  no,  or  very  little,  molecular  at¬ 
traction  for  water,  and  to  the  other  a  strong  molecular  attraction. 

Grout1  may  be  quoted  as  follows: 

“The  attraction  of  two  grains  may  vary  with  the  nature  of  the  grains.  The 
greater  the  attraction  the  farther  they  can  be  separated  without  losing  coher¬ 
ence.  - -  — .  Another  way  in  which  the  films  become  viscous  is  the 

result  of  molecular  attraction,  which  binds  a  film  over  the  surface  of  the 
grain  and  renders  it  viscous.  The  friction  between  this  film  and  the  solid 
grain  of  clay  is  said  to  be  infinite,  compared  with  water  outside  of  the  film. 
But  when  forced  to  move,  the  resistance  would  depend  on  the  strength  of 
the  attraction  of  clay  and  liquid. - .  The  change  in  viscosity  or 

1  Jour.  Am.Chem.Soc.,  Vol  .XXVII,  No. 9, Sept.  1905, p.1016. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK. 


179 


in  thicknes  of  the  film,  seems  to  be  beyond  the  region  of  experiment.  The 
quantity  is  too  small  to  admit  the  determination  of  slight  changes,  but  such 
are  constantly  assumed  in  physical  problems.  W.  J.  A.  Bliss  speaks  of  clay 
particles  and  the  surrounding  adherent  liquid  as  follows:  ‘The  thickness 
of  this  shell  depends  on  the  intensity  of  the  attraction  between  the  solid 
and  the  liquid.’  J.  E.  Mills  says:  ‘Molecular  attraction  depends  primarily 
on  the  chemical  constitution  of  the  molecule.  —  — Certain  rare 
organic  colloids  increase  the  plasticity  by  rendering'  the  water  viscous. 

- .  The  tendency  for  tensile  strength  to  vary  with  plasticity  is 

also  easily  explained  in  this  way.  Molecular  attraction  between  two  kaolin 
grains  may  be  high.  If  the  attraction  for  water  is  high,  some  water  will 
be  drawn  between  the  grains  and  rendered  viscous  by  the  attraction;  this 
makes  plasticity  high.  But  when  the  water  dries  out  from  such  a  mass, 
the  kaolin  grains  still  attract  each  other,  and  the  chances  are  for  greater 
strength  •  than  when  wet,  because  the  water  has  acted  as  a  lubricant,  allow¬ 
ing  a  readjustment  of  grains  to  fill  the  space  left  as  the  water  moved  out. 
The  result  is  a  high  degree  of  consolidation.” 

Mr.  Grout’s  arguments  may  be  summed  up  as  follows: 

1.  Attraction  varies  with  the  nature  of  grain,  i.  e.,  their  chemical  con¬ 
stitution,  or  in  other  words,  molecular  structure. 

2.  Films  become  viscous  as  a  result  of  molecular  attraction,  the  more 
strongly  attracted  film  being  .the  more  viscous. 

3.  Organic  colloids  increase  plasticity  by  rendering  the  water  film  viscous. 

4.  The  tendency  for  tensile  strength  to  vary  with  plasticity  is  explained 
by  molecular  attraction  between  grains. 

5.  Change  in  viscosity  or  in  thickness  of  film  is  beyond  the  region  of  ex¬ 
periment. 

Granting  that  these  arguments  may  be  valid  and  may  be  substan¬ 
tiated  by  facts,  it  will  be  shown  later  that  they  may  be  considered  as 
establishing  the  existence  of  an  effect  rather  than  the  existence  of  a 
cause. 

Size  of  Grain  Theory  of  Plasticity — It  has  been  shown  earlier  in  this 
discussion  that  the  size  of  the  grains  as  determined  in  the  mechanical 
analysis  does  not  agTee  with  the  normal  fineness  of  grain  in  the  clay  as 
it  issues  from  the  pug-mill;  there  are  bundles  of  grains  that  success¬ 
fully  withstand  the  disintegrating  effect  of  water  in  the  pugging  pro¬ 
cess,  but  which  are  to  a  large  extent  disintegrated  in  the  process  of 
mechanical  analysis.  It  is  obvious  therefore  that  conclusions  based 
wholly  on  the  results  obtained  in  the  mechanical  analysis  cannot  be 
considered  as  necessarily  agreeing  with  the  facts  observed  in  the  actual 
behavior  of  the  clay  under  factory  conditions.  In  many  clays,  however, 
these  bundles  are  broken  down  to  such  an  extent  that  the  analytical 
results  indicate  quite  accurately  their  actual  working  properties. 

Because  in  the  mechanical  analysis  the  coarser  grains  have  been  re¬ 
ported  as  sand  and  the  finer  particles  as  silt  and  clay,  not  a  few  have 
been  led  to  conclude  that  clay  particles,  or  at  least  particles  in  which 
clay  substances  constitute  a  large  proportion,  cannot  be  present  in  a 
clay  as  large  grains  after  thorough  disintegration  in  water.  Grout 
has  shown,  however,  that  this  conception  is  entirely  erroneous.1  In 
Table  XYII  is  given  the  amount  of  clay  substance  that  he  obtained  first 
from  the  analytical  analysis,  second,  calculated  from  ultimate  analysis, 
and  third,  obtained  from  mechanical  analysis. 


1  W.Va.Geol.Surv.,  Vol. Ill,  1905,  p.  26. 


180 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


Table  XVII. 


Showing  the  discrepancy  in  the  reported  “  clay  substance”  in  clay,  by  the 
three  methods  for  its  determination  now  in  vogue. 


Specimen  Number. 

(Rational  Analysis 

1 

Calculated  Kaolin 

Mechanical 

Analysis. 

4 . 

67.23 

52.30 

11.8 

17 . 

36.80 

26.39 

36.85 

41 . 

72.26 

41.65 

63.70 

62 . 

70.48 

41.14 

59.70 

76 . . 

42.41 

31.50 

33.35 

Mr.  G-rout  has  also  given1  results  of  the  chemical  analysis  of  a  com¬ 
plete  mixture  of  the  several  grades  of  fineness  obtained  from  16  samples 
of  clay  as  follows : 


Table  XVIII. 


Constituents. 

.00  to  .001 

.001  to  .005 

.005  to  .02 

.02  to  0.15 

0.15  up. 

Si02 . 

44.08 

54.54 

70.30 

81.16 

73.63 

ai2o3 . 

28.16 

23.00 

16.04 

9.76 

13.01 

Fe203  . 

7.94 

5  91 

3.21 

2.13 

4.71 

FeO . 

0.99 

0.99 

0.63 

0.40 

0.18 

MgO . 

1.36 

1.02 

0  80 

0.39 

0.48 

CaO . 

0.76 

0.82 

0.72 

0.31 

0.47 

Na20 . 

0.00 

0.29 

0.45 

0.56 

0.00 

KoO . 

3,05 

3  31 

2.14 

1.78 

0.93 

h2o . 

2.80 

1.10 

•0.56 

0.35 

0.87 

Ignition . 

10.86 

7.79 

4.33 

2.59 

4.40 

Ti02 . 

0.84 

1.12 

1.08 

0.78 

0.60 

In  this  he  has  proved  conclusively  that  the  “clay  substance”  is  pres¬ 
ent  in  every  grade  of  fineness.  His  own  conclusions  from  these  analyses 
are,  however,  rather  startling.  He  says:  “The  silica  percentage  is 
higher  in  the  coarser  portions,  where  it  probably  is  present  in  the  form 
of  sand  or  quartz.  Alumina  is  higher  in  the  finer  material,  but  total 
fluxes  are  also  higher,  so  that  the  finest  particles  are  not  the  purest’ 
kaolin.” 

In  order  better  to  show  the  validity  of  his  conclusions  his  data  has 
been  calculated  into  molecular  equivalents  as  given  in  the  following 
table : 


1W.  Va.  Genl.  Surv.,  Vol.  Ill,  p.  61. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


181 


Table  XIX. 


Grades  of  Fineness. 

Si02 

ai2o3 

Fe203 

FeO 

MgO 

CaO 

Na20 

k2o 

Ti02 

0  00  to  0  001 . 

2.66 

1.00 

0.18 

0.05 

0.12 

0.05 

0.12 

0.04 

0.001  to  0.005 . 

4.03 

1.00 

0.16 

0.06 

0.11 

0.06 

0.02 

0.16 

0.06 

0.005  to  0.02 . 

7.45 

1.00 

0.13 

0.06 

0.13 

0.08 

0.05 

0.14 

0.09 

0.02  to  0.15 . 

14.14 

1.00 

0.14 

0  59 

1.02 

0.58 

0.94 

0  19 

1.02 

0.15  up . 

9.62 

1.00 

0.02 

0.002 

0.01 

0.007 

0.008 

0.006 

A  review  of  Grout’s  mechanical  analysis  of  the  West  Virginia  clays 
discloses  the  fact  that  he  made  26  determinations : 

6  plastic  fire  clays,  pp.  160,  162,  163,  233  and  251, 

1  flint  fire  clay,  p.  218, 

7  shales,  pp.  249,  251,  242  and  262, 

10  river  clays,  pp.  263,  265,  270,  272,  274,  and  276. 

1  glacial  clay,  p.  265, 

1  residual  surface  clay,  p.  200. 

It  is  assumed,  therefore,  that  the  samples,  the  analyses  of  which  are 
given  in  Table  XVI,  are  composites  of  the  several  grades  of  grains  from 
the  above  clays.  Being  in  most  cases  very  impure  clays,  it  is  con¬ 
sidered  that  although  a  study  of  the  possible  mineral,  make-up  of  each 
grade  is  at  the  best  largely  based  on  hypothetical  assumptions,  such  a 
study  would  aid  in  our  attempt  to  understand  the  constitutional  make¬ 
up  of  our  clays. 

On  the  assumption  that  all  the  alkali  is  present  as  a  RO  in  orthoclase 
feldspar,  the  molecular  ratio  and  ratio  by  weight  of  kaolin,  feldspar 
and  quartz  present  in  each  grade  would  be  as  follows: 

Table  XX. 


Showing1  possible  mineral  constitution  of  the  several  grades  of  grains  in 

impure  clays. 


Grade. 

Molecular  Ratio. 

Weight  Ratio. 

Quartz. 

Kaolin. 

Feldspar. 

Quartz. 

Kaolin. 

Feldspar. 

0.00  -0.001 . 

0.88 

0.12 

.18 

10 

2.9 

0.08 

0.001-0.005 . 

.82 

0.18 

1.25 

10 

4.7 

3.5 

0.005-0.02 . 

.81 

0.19 

4.69 

10 

5.1 

13.5 

0  02  -0  15 . 

1.00 

8.14 

10.0 

8. 

0.1S  up . 

0  992 

0.008 

3.62 

10 

0.18 

8.5 

This  data  checks  the  fact  developed  in  Table  XV,  i.  e.,  that  clay  sub¬ 
stance  is  to  be  found  in  all  of  the  grades  of  fineness,  in  the  coarsest  as 
well  as  the  finest.  It  also  shows  that  more  than  50  per  cent  of  the 


182 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


coarsest  group,  or  as  it  is  customarily  called,  "coarse  sand,”  may  be 
kaolin,  or  is  at  least  kaolinitic  in  composition. 

As  a  further  analysis  of  the  probable  mineral  make-up  of  clays. 
Grout’s  data  will  be  discussed  by  groups.  In  this  only  the  most  com¬ 
mon  and  abundant  minerals  known  to  occur  in  clays  are  considered. 

Coarsest  grade  (0.15  mm.)  :  This  grade  of  grain,  even  if  all  the 
alkali  is  considered  as  being  present  as  a  constituent  part  of  feldspar 
grains,  would  be  assumed  to  be  composed  almost  entirely  of  non-dis- 
integrated  kaolin  and  quartz  grains.  Only  in  one  case,  however,  does 
Mr.  Grout1  speak  of  the  physical  character  of  the  grains  of  this  grade. 
In  this  particular  case  the  clay  examined  is  a  shale.  "The  12.9  per 
cent  (referring  to  coarse  sand  grade)  on  3  mm.  screen  was  mostly  flat 
scales  of  shale,  about  5  mm.  in  size,  of  red  and  greenish  color.”  The 
total  absence  of  similar  description  of  this  grade  in  the  other  25  samples 
justifies  the  conclusion  that  the  grains  of  this  grade  were  flat  or  scale 
like  only  in  this  one  sample.  If  this  conclusion  is  true,  then  it  is  fair 
to  assume  that  either  the  kaolin  scales  are  present  in  undissolvable 
bundles,  or  these  grains  are  not  composed  of  kaolin  but  some  other 
aluminum  compound  like  gibbsite,  etc. 

On  the  other  hand,  it  is  hardly  possible  that  grains  of  feldspar  of 
this  size  could  remain  unaltered  in  these  old  river  clays  that  have  been 
elutriated,  mixed  and  moyed  by  fresh  waters  possibly  for  ages.  There 
is  justification  for  the  assumption,  therefore,  that  these  coarse  grains 
are  bundles  of  kaolinitic  grains  cemented  together  so  tightly  by  some 
salt  that  they  resist  disintegration  by  water.  If  the  alkalies  had  been 
present  as  constituent  parts  of  feldspar  grains  of  this  size,  the  feldspar 
crystals  could  have  been  easily  recognized  under  the  microscope  as 
cubical  grains  and  not  flat  scales. 

H.  B.  Fox,  in  the  Ceramic  laboratories  of  the  University  of  Illinois, 
separated  the  grains .  of  a  shale  and  a  glacial  clay  into  the  several 
grades  of  fineness,  and  found  that  all  the  grades  possessed  a  plasticity 
that  varied  directly  with  the  fineness  of  grain,  and  that  the  coarse 
grains  which  could  not  be  disintegrated  by  20  hours  of  constant  shaking 
in  water,  when  broken  down  in  a  mortar,  developed  plasticity  that  in¬ 
creased  as  the  size  of  the  grains  decreased,  until  when  the  coarse  grains 
had  been  reduced  to  an  impalpable  powder  they  developed  a  plasticity 
nearly  equal  to  that  exhibited  in  the  finest  grains  that  had  been  separ¬ 
ated  from  the  original  sample,  showing,  it  is  believed,  that  the  coarser 
grains  were  comprised  of  materials  similar  in  every  respect  to  those  in 
the  fine  grains,  but  cemented  in  such  a  way  that  they  withstood  success¬ 
fully  the  disintegration  treatment. 

(0.02  to  0.15)  grade:  It  is  highly  improbable  that  this  grade  con¬ 
tained  no  kaolin  or  clay  substance,  but  such  would  have  to  be  the  case 
if  all  the  alkali  was  present  as  a  constituent  part  of  the  orthoclase  feld¬ 
spar  grains.  The  alkalies  cannot  be  present  in  this  case,  as  easily  soluble 


iw.  Va.  Genl.  Surv.,  Vol.  Ill,  p.  249. 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  18B 

salts,  for  the  alkaline  salts  would  have  been  dissolved,  carried  in  solu¬ 
tion,  and  would  affect  only  the  finest  grades.  If  the  feldspar  was  oligo- 
clase  and  not  orthoclase,  then  the  0.5  equivalents  of  the  alumina  could 
be  considered  as  a  constituent  of  kaolin  grains. 

Although  there  is  no  statement  made  as  to  the  presence  of  mica  in 
the  clays  from  which  these  grades  of  grains  were  obtained,  Mr.  Grim- 
sley* 1  ‘states  that  it  is  a  very  common  constitutent  of  the  West  Virginia 
clays.  Stull2  gives  as  the  chemical  formula  of  common  muscovite  mica 
the  following : 

0.1243  CaO . -..1 

0.1103  MgO . !  1.000  A1203  . )  6.399  Si02— 0  1)74  H20 

0.3280  K20 .  [0.1857  Fe203  . (Comb.  Wt.  582.167 

0.0929  Na20 . J 

On  the  assumption  that  the  alkali  in  this  grade  is  derived  wholly  from 
muscovite  mica  of  the  composition  given  by  Stull, -the  mineral  constitu- 
tents  of  this  grade  of  grain  might  be  proportioned  as  shown  by  the 
following  calculations : 


.57  Eqv.  Mica 

Si02 

14.14 

3.65 

10.49 

.86 

A1203 

1.00 

0.57 

0.43 

0.43 

Fe203 

0.14 

0.11 

0.03 

FeO 

0.59 

MgO 

1.02 

0.06 

0.96 

CaO 

0.58 

0.07 

0.51 

Na20 

0  94 

0.05 

0.89 

K20 

0.19 

0.19 

Ti02 

1.02 

43  Eqv.  Kaolin. . . . 

0.59 

1.02 

.63 

0.89 

0.57  Eqv.  Mica  x  582.167  =  331.835  or  by  proportion  30.0 

0.43  Eqv.  Kaolin  x  258  =  110.940  or  by  proportion  10.0 

0.63  Eqv.  Silica  x  60  =  37.800  or  by  proportion  3.4 

In  this  case,  the  formula  most  favorable  to  the  supposition  that  all 
the  IGO  is  present  in  the  form  of  mica  has  been  taken.  If  the  theoret¬ 
ical  formula  IGO,  3  AbCh,  6  Si  Cb,  2  ILO  had  been  taken,  there  would 
have  been  either  considerable  IGO  to  account  for  in  some  other  way,  or 
return  to  the  original  hypothesis  that  this  group  contained  no  kaolin. 
Either  supposition  leaves  considerable  alkali  unaccounted  for,  which 
as  has  been  pointed  out,  could  not  possibly  be  present  in  an  early  soluble 
form. 

The  supposition  therefore  that  this  grade  is  composed  in  part  of 
kaolinitic  grains  cemented  together  by  some  alkaline  salts,  finds  sup¬ 
port  in  any  plausible  assumption  that  may  be  made. 

(0.005 — 0.02  and  (0.001 — 0.005)  groups:  If  the  kaolin  grains  in 
these  groups  were  in  their  natural  condition,  i.  e.,  flat  plate-like  crystals> 
they  should,  theoretically,  be  visible  through  the  microscope.  This  evi¬ 
dently  was  not  the  case.  Beyer  and  Williams1  say:  “While  it  is  next 
to  impossibleto  make  out  much  concerning  the  crystalline  character  of 
the  minerals,  it  is  also  difficult,  because  of  their  minute  size  in  most 
secondary  clays,  to'  say  anything  regarding  their  shape/’ — In  Other 


1W.  Va.  Geol.  Surv.,  Vol.  Ill,  p.  12. 
2  A.  C.  S.  Trans.  Vol.  IV,  p.  258. 

1  la.  Geol.  Surv.,  Vol.  XIV,  p.  94. 


184 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


words,  the  shape  of  the  grains  is  irregular  and  non-conformable  one 
with  another.  If  in  these  grades  there  is  as  much  kaolin  as  is  shown  in 
Table  XVIII,  its  grains  must  be,  to  a  very  great  extent,  in  bundles.  If 
feldspar  or  mica  is  present  in  such  amount  and  size  of  grain,  as  the  cal¬ 
culated  data  suggest,  their  grains  ought  to  be  detectable  with  the  aid 
of  the  microscope.  Such,  however,  is  not  the  case.  The  supposition, 
therefore,  that  all  the  akalies  of  these  two  grades  are  there  as  constitu¬ 
ent  parts  of  feldspar  and  mica  is  certainly  untenable. 

(0.000-0.0001)  grade:  The  molecular  composition  of  this  group  is 
certainly  very  instructive.  That  in  such  heterogeneous  mixtures  as 
shales  and  river  clays  the  finest  particles  are  found  to  be  composed  in 
the  main  of  kaolinitic  grains  is  certainly  astonishing.  If  the  bases 
present,  as  shown  in  Table  XIX,  page  181,  are  considered  as  being 
present  as  soluble  salts  that  were  either  originally  present  in  the  clays 
or  in  part  introduced  during  the  process  of  analysis,  (a  most  plausible 
assumption)  then  there  would  remain  but  one  conclusion,  that  is,  that 
the  finest  insoluble  grains  are  almost  entirely  kaolinitic  in  composition. 

Taking  data  given  by  Grout,  it  was  calculated  that  if  all  the  soluble 
salts  originally  in  the  clays  were  in  the  finest  group,  they  would  amount 
to  2.7  per  cent  of  the  weight  of  that  group.  The  2.7  per  cent,  together 
with  the  soluble  salt  introduced  during  the  process  of  analysis  from 
glassware,  water,  atmospheric  dust,  etc.,  would  account  for  nearly  all 
of  the  alkali  in  the  finest  grade.  It  is  not  mere  assumption  therefore, 
that  the  finest  particles  in  clay,  contrary  to  Grout’s  statement,  are  the 
purest  kaolin  grains. 

In  the  course  of  the  research  on  paving  brick  clays  by  the  survey  there 
was  much  speculation  as  to  the  number  of  these  submicroscopic  kaolin 
grains  in  the  various  shales.  This  was  readily  ascertained  as  follows: 
By  dividing  the  percentage  amount  of  the  group  (0.001  to  0)  by  100, 
and  considering  that  as  being  a  part  of  1  milligram  of  the  sample,  (for 
the  size  of  the  particles  is  in  millimeters)  then  dividing  this  amount  by 
the  specific  gravity  of  the  clays,  a  figure  is  obtained  that  represents  the 
sum  or  total  volume  in  cubic  millimeters  of  the  particles  comprising 
the  group.  Considering  0.0005  as  the  mean  diameter  of  the  particles, 
6 

by  the  formula  - the  volume  of  each  particle  is  found  to  be  654xl0'13 

PiD3 

cubic  millimeters.  Then  for  each  day,  bv  dividing  the  total  volume 
of  the  particles,  by  the  volume  of  one  particle,  the  number  of  grains 
per  milligram  of  the  sample  will  be  obtained.  By  multiplying  the  num¬ 
ber  of  grains  in  one  milligram  by  1,000  there  would  be  obtained  the 
number  of  grains  in  1  gram;  or  by  multiplying  by  352,740  there  would 
be  obtained  the  number  of  grains  of  this  size  in  1  oz.  of  the  whole 
sample.  In  this  way  Table  XXI  was  calculated. 

1  This  mean  diameter  is  twice  as  large  as  that  given  by  Whitney  for  the  finest 
group;  U.  S.  Dept,  of  Agr.  Weather  Bureau,  Bull.,  No.  4,  p.  35. 


PURDY]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  185 

Table  XXI. 


Number  of  grains  of  group  (0.001  to  0)  in — 


Sample  No. 

1  gram  of  the  clay. 

1  oz.  of  the  clay. 

K  1 . 

560  trillions. 
829.0  trillions. 
794.0  trillions. 
1,588.7  trillions. 
1,940.0  trillions. 
440.0  trillions. 

197, 534  trillions. 
292,421  trillions. 
280,075  trillion4. 
560, 398  trillions. 
684,315  trillions. 
166, 205  trillions. 

K  2 . 

K  3  . 

H  23 . 

H  21 . 

K  6  . 

These  figures,  although  beyond  the  limits  of  perception  of  the  human 
mind,  are  not  larger  than  the  figures  representing  the  countless  germs 
that  bacteriologists  claim  can  exist  in  a  single  drop  of  a  fluid.  Startling 
as  this  data  appears  to  be,  it  cannot  be  other  than  true  if  the  analytical 
results  of  the  mechanical  separation  are  correct. 

If  these  minute  particles  were  not  kaolin  grains,  would  they  add  to 
the  real  plasticity  of  the  clay?  Potter’s  flint  (dry  ground)  is  finer 
grained  than  most  clays,  and  particularly  more  so  than  the  shales,  yet 
it  does  not  exhibit  the  faintest  sign  of  plasticity.  Orton1  found  that 
glass  particles  which  were  so  fine  that  they  remained  in  suspension  for 
hours  without  settling,  when  collected  exhibited  no  plasticity.  Wheeler2 
found  that  while  quartz  crystals  ground  to  200  mesh,  seemed  to  be  ap¬ 
preciably  plastic,  on  drying  the  coherence  was  so  slight  that  it  required 
the  gentlest  .handling  to  prevent  the  molded  sample  from  falling  to 
pieces.  Fine  quartz  dust .  and  impalpable  geyserite  or  finely  precipi¬ 
tated  opal,  dried  to  a  very  tender  mass.  The  same  was  true  of  tripoli. 
Wheeler3  found  that  some  plasticity  could  be  developed  in  powdered 
slate,  prophylite,  talc,  gypsum,  halloysite,  etc.,  but  that  the  plasticity 
developed  was  only  apparent  plasticity,  except  perhaps  in  the  case  of 
slate.  The  powdered  gypsum  when  molded  and  dried  formed  a  rela¬ 
tively  hard  mass,  but  this  hardness  would  be  expected  on  account  of  the 
solubility  of  gypsum  in  water.  The  plasticity  of  the  slate,  which  is  a 
dehydrated  shale  has  caused  considerable  surprise,  and  has  strengthened 
the  fineness  of  grain  theory  of  plasticity.  That  powdered  slate  should 
develop  plasticity  need  not  be  so  great  a  source  of  wonder,  for  in  the 
course  of  the  Survey  work  a  shale,  after  having  been  held  at  heat  rang¬ 
ing  from  500°  to  800°  C.  for  17  hours,  slaked  down  in  water  to  a 
red  plastic  mass  in  the  same  manner  as  the  unburned  shale  at  the  bank. 
True,  the  plasticity  of  this  partially  burned  shale  was  not  equal  to  the 
plasticity  shown  by  the  clay  before  dehydration,  but  its  plasticity  was 
considerably  more  than  that  of  some  of  the  harder  shales  before  being 
burned.  Fineness  of  grain  in  itself  then  does  not  seem  to  be  the  cause 
of  plasticity.  It  may  said,  however,  to  be  a  required  condition  in  the 
operation  of  the  real  cause. 


1  Brick,  Vol.  XIV,  No.  4,  p.  216. 

2  Mo.  Geol.  Surv.,  Vol.  XI,  p.  102. 

3  Loc.  cit.,  p.  106. 


186 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


In  seeming  contradiction  to  this  statement  regarding  the  fineness  of 
grain  as  a  sense  for  plasticity,  is  the  fact  that  finer  grinding  of  given 
clay  increases  its  plasticity;  but  quoting  Wheeler1:  “While  it  is  true 
that  fine  clays  are  usually  very  plastic  and  coarse  clays  much  less  so, 
there  are  very  many  exceptions.”  And  again,  Grout2  says  that  while 
the  majority  of  clays  improve  on  fine  grinding,  some  are  unchanged. 

Wheeler3 * 5  reports  the  physical  structure  of  a  few  clays  as  follows: 

Moberly  shale  (400  diarn.) : 

Mainly  clusters  of  thick  plates  with  minor  portions  split  off;  moderately 
plastic;  suggests  fine  grinding  to  develop  plasticity. 

Aldrich  shale  (325  diam.): 

One-third  dolomite  crj^stals;  bulk  in  coarse  thick  crystals  or  plates; 
rest  in  fine  state  of  division;  moderately  plastic. 

Unweathered  Leasburg  flint  fire  clay  (950  diam.): 

Almost  all  fine  particles;  no  plates  or  scales;  devoid  of  plasticity. 
Weathered  Leasburg  flint  fire  clay  (950  diam.): 

Numerous  coarse  plates  present  and  occasionally,  apparently  a  few  thin 
plates.  Came  from  same  bank  the  same  day  a  few  feet  from  the 
unweathered  sample. 

Hartwell  loess  clay  (400  diam.) : 

Large  angular  fragments  which  were  undoubtedly  sand,  and  apparently 
some  clusters  of  plate  crystals,  with  only  a  minor  portion  of  small 
plates;  very  plastic. 

There  is  sufficient  evidence  in  the  above  citations  to  show  that  any 
theory  so  far  discussed  other  than  that  of  molecular  attraction,  is  in¬ 
sufficient  to  account  for  the  presence  or  absence  of  plasticity. 

Plate  Structure  Theory  of  Plasticity — Grout1  has  recorded  the  fact 
that  in  the  case  of  the  Thornton  Brick  Company’s  plastic  clay  the 
amount  by  weight  of  the  particles  below  0.005  mm.  in  diameter  rose 
from  7.7  per  cent  to  17.8  per  cent  by  weight  in  one  wetting  and  drying. 
Fox,  in  our  laboratories,  found  that  the  plates,  although  not  disintegrat¬ 
ed  by  twenty-four  hours  of  shaking  in  water,  would  break  down  by 
mechanical  crushing  or  by  disintegration  in  acids  and  caustic  alkali, 
and  that  when  so  broken  down  the  mass  became  considerably  more 
plastic.  Wheeler1  not  only  advises  fine  grinding  in  the  case  of  the 
Moberly  shales,  but  relates  a  most  remarkable  instance  of  a  clay  in 
which  the  grains  on  weathering  formed  themselves  into  clusters  re¬ 
sembling  plates.  It  seems  highly  probable,  therefore,  that  these  plates 
or  coarse  grains  are  bunches  or  bundles  of  minute  grains  cemented  to¬ 
gether  by  salts  that  are  to  a  greater  or  less  extent  soluble  in  water,  and 
that,  depending  upon  the  solubility  of  the  cementing  salt  in  a  particular 
case,  or  the  peculiar  compactness  of  the  grains  in  another,  it  requires 
a  greater  or  less  amount  of  time  to  cause  a  breaking  down  of  these 
bundles.  It  can  be  readily  conceived  that  the  adsorptive  power  of  the 
particles  when  combined  with  their  axes  in  a  certain  general  direc¬ 
tion,  for  instance,  has  greater  power  in  holding  certain  of  these  cement¬ 
ing  salts  than  the  solvent  action  exerted  by  the  water  can  overcome. ' 
The  solvent  power  of  water,  in  other  words,  is  not  sufficient  to  overcome 
the  adsorptive  power  of  the  kaolinitic  grains. 

l  Loc.  cit.,  p.  109. 

2W.  Va.  Geol.  Surv.,  Vol.  Ill,  p.  46. 

3  Loc.  cit.,  pp.  104  to  109. 

4W.  Va.  Geol.  Surv.,  Vol.  Ill,  p.  46. 

5  Loc.  cit.,  p.  105. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


187 


These  coarse  grains  add  to  the  plasticity  of  the  clay  as  a  whole  in  a 
-ratio  to  the  surface  exposed.  Every  exposed  kaolin  particle  is  as  effec¬ 
tive  in  enhancing  plasticity  as  the  very  small  independent  particles.  The 
extent  to  which  the  larger  grains  would  affect  plasticity  would,  there¬ 
fore,  be  in  proportion  to  the  exposed  surface  of  the  particles  of  which 
the  bundle  or  cluster  is  composed. 

1  Further,  it  is  fair  to  challenge  the  plate  theorist  to  demonstrate  that 
these  small  grains  when  cemented  together  in  a  bundle  or  cluster  have 
not  a  tendency  to  line  up  one  with  another  so  that  their  longest  axes 
1  will  lie  in  the  same  relative  plane,  just  as  they  are  in  the  natural  kaolin 
crystals,  i.  e.,  in  plate  forms.  The  plate  theorist  must  admit  that  when 
these  bundles  are  thus  formed  they  are  well-nigh  indistinguishable  from 
mica  crystals,  and  that  the  very  large  majority  of  so-called  plates  or 
scales  of  kaolin  in  a  clay  are  most  likely  to  he  mica.  It  is  certainly 
strange  that  on  one  page  of  a  report  there  will  be  a  statement  to  the 
effect  that  “the  clays  of  this  state  are  quite  micacious,”  and  another 
page  will  report  the  scales  -that  appear  on  the  stage  of  the  micro¬ 
scope  as  “kaolin  scales  or  plates.” 

'  If  the  idea  that  has  been  put  forward  in  the  foregoing  is  correct, 
then  we  must  agree  with  Dr.  Ladd1  when  he  says:  “The  question  of 
fineness  of  grain  and  shape  of  the  particle  becomes,  then,  largely  but 
modifying  factors,  affecting  degree,  and  being,  within  large  limits  at 
least,  modifiers,  rather  than  determinants  of  plasticity.” 

1  It  is  quite  evident  that  the  peculiar  physical  make-up  of  a  kaolin 
grain,  so  far  as  the  eye  by  the  aid  of  the  microscope  can  discern,  is  not 
fundamentally  responsible  for  their  individuality,  as  expressed  in  their 
power  to  develop  plasticity.  If  the  structure  of  the  grains  which  en¬ 
ables  a  mass  of  them  to  develop  plasticity  is  not  detectable  by  the  micro¬ 
scope,  direct  observation  and  measurement  are  obviously  inadequate  in 
finding  the  true  cause. 

*  Pectoidal  Theory  of  Plasticity — Turning  to  indirect  or  circumstantial 
evidence,  there  are  many  facts  observed  by  a  good  many  careful  scientists 
that  seem  to  point  to  one  thing  that  is  more  characteristic  of  kaolin 
grains  than  of  any  other  of  the  inorganic  substances  or  minerals  of 
which  a  clay  is  composed,  i.  e.,  adsorptive  power.  Some  investigators 
have  even  gone  so  far  as  to  attribute  the  plasticity  of  kaolin  grains  to  an 
adsorptive  power  or  actual  taking  into  the  grains  themselves  of  foreign 
salts  from  solution.  They  advance  the  theory  that  these  minute  grains 
have  a  micellian  structure.  To  such  substance  they  apply  the  name 
“Pectoid,”  and  to  the  theory  the  name,  “Pectoid  theory.”  To  many,  the 
absorptive  and  adsorptive  properties  of  a  clay  are  one  and  the  same 
thing,  and  so  far  as  can  be  judged,  the  most  radical  believe  in  either 
the  adsorptive  or  the  pectoidal  theory,  and  oscillate  from  one  to  the 
other  in  a  manner  that  induces  skepticism.  The  fact  remains,  how¬ 
ever,  that  both  use  the  same  arguments,  the  only  difference  being  in  the 
conception.  It  is  safe  to  warrant,  that  when  the  pectoid  theorist  real- 


l  Geol.  Surv.,  of  Ga.,  Bull.  6-A,  p.  32. 


188 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


izes  than  in  one  gram  of  clay,  the  disintegration  of  which  has  been 
effected  only  by  shaking  in  distilled  water,  there  can  exist  from  400 
to  1500  trillion  free  and  independent  sub-microscopic  particles,  to  say 
nothing  about  the  larger  particles,  they  will  find  interstices  between 
these  grains  sufficient  to  satisfy  even  the  most  exaggerated  conception  of 
a  micellian  structure. 

1  Dr.  Cushman1  in  a  brief  review  of  the  observations  that  point  toward 
a  colloidal  substance  as  being  the  prime  cause  of  plasticity,  has  given 
the  following  citations :  “Daubree  found  that  wet  ground  feldspar  as¬ 
sumed  a  plastic  condition,  whereas  dry  ground  feldspar  did  not.”  Ost- 
wald,  the  eminent  German  physical  chemist,  Arons,  Bischol,  Seger, 
Rokland,  and  Van  der  Bellen,  accepted  and  advanced  in  substance  the 
colloid  theory.  T.  Way2  stated  that  while  particles  of  sand  and  chalk 
absorbed  water,  owing  to  surface  attraction  and  capillarity,  clays  and 
soils  with  a  clay  base  behaved  in  a  quite  extraordinary  manner.  The 
more  clayey  the  soil  the  more  water  it  seemed  capable  of  absorbing. 
But  this  was  not  all;  besides  water  this  clay  substance  exhibited  a 
greater  facility  for  absorbing  the  bases  contained  in  certain  salts  which 
Were  dissolved  in  the  water.” 

1  E.  Bourry2  says  that  if  clay  is  mixed  with  a  solution  of  calcium  car¬ 
bonate,  the  clay  will  retain  some  of  the  carbonate.  “Kaolins  do  not 
retain  more  than  2  per  cent  of  carbonate  of  lime  in  solution,  while 
plastic  clays  can  absorb  from  10  to  20  per  cent  of  it.” 
i  It  is  common  knowledge  among  chemists  that  clay  can  extract  solu¬ 
ble  salts  from  solution  and  retain  them  very  persistently  against  all 
(attacks  by  dissolving  mediums.  Mr.  Ackison3  found  that  catechu  and 
extract  of  sumac  leaves,  spruce  bark,  tea  leaves,  oak  bark  or  straw 
would  be  absorbed  by  clays  from  solutions. 

*  Further,  Ries4 5  advanced  the  theory  that  the  action  of  hydro-carbons 
in  solution  was  to  deflocculate  the  particles.  This  he  claims  was  proved 
by  the  fact  that  in  the  untreated  Clays  the  grains  were  bunched  to¬ 
gether  while  in  the  treated  clays  the  particles  were  separated. 

1  In  the  foregoing  citations  there  is  evidence  sufficient  to  formulate  a 
Conception  of  what  changes  have  taken  place  in  a  clay  from  the  time 
it  was  first  formed  in  situ  by  decomposition  of  the  parent  rock  and 
left  practically  devoid  of  plasticity,  until  it  was  deposited  elsewhere  as 
a  plastic  clay.  Organic  matter  would  have  deflocculated  the  particles, 
(and  the  soluble  salts,  which  are  very  naturally  attracted  to  the  kaolin 
grains,  would  soften  when  wetted,  but  the  water  could  not  extract  them 
owing  to  the  greater  force  exerted  by  the  kaolin  particles.  Defloccul^- 
tion  by  organic  matter,  recementation  by  salts  of  various  kinds,  may 
have  formed  a  cycle  of  events  that  in  the  end  would  cause  a  condition 
Of  affairs  that  makes  possible  the  property  described  as  plasticity. 

Molecular  attraction  for  foreign  substances  which  is  peculiar  to  kaolin 
particles,  and  not  to  a  very  large  degree  to  other  common  constituents 
of  clay,  may  and  does  have  its  influence  on  plasticity  affected  by  fine- 

1  Vol.  VI,  A.  C.  S.,  p.  66. 

2  Royal  Ag-.  Soc.,  Jour.  XI,  1850,  cited  by  Cushman. 

3  Emile  Bourry  ,  Treatise  on  Ceramic  Industries,  p.  54. 

4  Amer.  Cer.  Soc.  Trans.,  Vol.  VI,  p.  33. 

5  A.  C.  S.  Trans.,  Vol.  VI,  p.  43. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


189 


ness  of  grain.  Plate  structure,  a  natural  form  in  which  kaolin  grains 
arrange  themselves,  is  a  possibility  under  the  right  conditions,  but  there 
is  also  a  possibility  that  organic  matter  and  adsorbed  salts  may  operate 
in  the  destruction  and  formation  of  the  plate  grains.  A  clay  that  is  of 
secondary  origin,  like  our  common  clays,  could  not  have  passed  through 
the  many  geological  changes  with  which  they  are  credited  without  being 
more  or  less  defloeculated  and  saturated  with  these  foreign  substances. 
Micellian  structure  is  not  a  necessary  condition.  Minuteness  of  grain 
and  consequently  large  surface  or  adsorbing  area  is  sufficient. 

Adsorption  theory  of  plasticity — Existing  data,  accumulated  for 
years  by  scientists,  all  point  to  the  fact — which  is  almost  beyond  the 
theoretical  state,  lying  wholly  within  the  realm  of  experimentation — 
that  the  plasticity,  tensile  strength  and  general  working  properties  of 
the  clay  can  be  traced  back  to  the  adsorptive  property  of  kaolin.  Fur¬ 
ther,  all  the  facts  that  have  been  cited  in  support  of  any  and  all  of  the 
theories  are  identifiable  as  conditions  that  allow  of  the  fullest  exhibition 
Qf  the  plasticity  that  seems  to  follow  as  a  direct  consequence  of  the 
adsorption  of  soluble  organic  and  inorganic  substances  by  the  kaolin 
grains. 

DEVELOPMENT  OF  PLASTICITY  IN  THE  PRESENCE  OF  WATER. 

Whatever  may  be  the  fundamental  cause  of  this  phenomenon  we  call 
plasticity,  it  is  certain  that  it  is  manifested  only  when  water  is  present. 
It  has  been  shown  that  mere  molecular  attraction  between  the  clay 
grains  and  the  water  molecules  is  not  sufficient  to  account  for  plasticity. 
There  must,  therefore,  be  factors  other  than  molecular  attraction  which 
becomes  operative  in  developing  this  property,  which,  when  water  is ' 
not  present,  may  be  said  to  be  latent.  Since  it  is  the  presence  of  water 
that  makes  the  development  or  expression  of  plasticity  possible,  it  is 
important  that  we  consider  some  of  the  fundamental  and  well-known 
hydrostatic  forces. 

There  are  at  least  four  forces  operating  on  the  water  in  a  wet,  un¬ 
burned  brick:  First,  gravity,  or  the  weight  of  the  water  itself;  second, 
surface  tension,  which  is  due  to  attraction  (cohesive)  between  the 
molecules  of  water  themselves;  third,  molecular  attraction  (adhesive) 
between  the  water  molecules  and  the  mineral  particles  in  the  clay;  and 
fourth,  surface  pressure,  which  is  the  opposite  of  surface  tension. 

Gravity — Surface  tension,  or  the  contracting  power  of  any  exposed 
water  surface,  may  act  with  gravity  or  against  gravity,  depending  upon 
circumstances.  Molecular  attraction  between  the  mineral  and  water 
molecules  always  acts  in  opposition  to  gravity.  Since,  as  can  be  shown, 
the  conditions  of  capillarity  in  a  mass  of  clay  compressed  into  the  form 
of  a  brick  is  such  as  to  make  surface  tension  the  very  much  greater 
force,  and  operating  in  opposition  to  that  of  gravity,  gravity  will  not 
be  considered  as  one  of  the  component  forces  in  our  problem.  If  we 
were  dealing  with  “slips”  or  even  soft  mud  fixtures,  the  force  of  gravity 
would  have  to  be  considered. 

Molecular  Attraction — Milton  Whitney1  says:  “The  potential  of  a 
single  water  particle  is  the  work  which  would  be  required  to  pull  it 


1  U.  S.  Dept,  of  Agr.  Weather  Bureau,  Bull.  4,  p.  19. 


190 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


away  from  the  surrounding  water  particles  and  remove  it  beyond  their 
sphere  of  attraction.  It  is  the  total  attraction  between  a  single  particle 
and  all  other  particles  which  surround  it.”  It  is  called  by  some  “mole¬ 
cular  attraction.” 

Surface  Tension — Because-  it  has  particles  adjoining  it  only  on  one 
side,  i.  e.,  molecular  attraction  is  affecting  it  only  from  one  side,  the 
potentiality  of  a  water  particle  on  the  surface  is,  according  to  Whitney’s 
definition,  only  one-half  that  of  a  particle  in  the  center  of  a  drop.  That 
things  tend  to  move  from  points  of  low  to  points  of  high  potential  is 
a  well-known  law  of  physics.  The  particles  on  the  surface,  will,  there¬ 
fore,  strive  to  get  to  the  interior  of  the  drop.  The  results  will  be  sur¬ 
face  tension. 

Looking  at  this  proposition  from  the  mechanical  point  of  view,  the 
force  of  molecular  attraction  operating  on  the' surface  particles,  is  effec¬ 
tive  along  lines  that  extend  from  the  center  of  each  particle,  to  the 
center  of  the  surrounding  particles.  Since  the  particle  on  the  surface 
of  a.  drop  of  water  is  under  the  influence  of  other  particles  only  from 
one  side,  the  several  lines  of  force  would  extend  radically  from  its  center 
to  the  center  of  adjacent  particles,  having  as  a  resultant  a  line  of  force 
extending  from  the  center  of  the  surface  particle  to  the  center  of  the 
mass. 

Surface  Pressure — Suppose  that  instead  of  a  drop  we  have  the  same 
mass  of  water  surrounding  a  solid  particle  as  a  film,  say,  0.0005  m  m. 
thick.  We  should  have  in  this  system  two  combating  forces,  first,  mole¬ 
cular  attraction  of  water  molecules  for  each  other,  causing  a  pull  on  all 
water  particles  toward  the  center  of  the  film,  creating  a  tension  on  the 
outside  surface  as  well  as  on  the  surface  contiguous  to  the  solid  par¬ 
ticles;  second,  attraction  between  the  molecules  of  the  solid  particles 
and  those  of  the  liquid,  tending  to  create  a  tension  only  on  the  outer 
surface  of  the  glm. 


Fig.  16.  Diagram  showing  operation  of  forces 
causing  surface  pressure. 


Consider  the  water  between 
four  solid  particles  as  shown  in 
the  following  figure  as  having  a 
potentiality  less  than  that  of  the 
solid  particles. 

All  water  particles  will  press 
outward  the  solid  particles  along 
the  resultant  lines  of  force  as 
shown  in  Fig.  16.  In  this  case 
instead  of  tension  we  would  have 
a  pressure.  This  pressure  is 
known  as  surface  pressure. 

If,  on  the  other  hand,  the 
water  had  a  potentiality  that  was 
greater  than  that  of  the  solid  par¬ 
ticles,  the  resultant  forces  of  at¬ 
traction  would  be  toward  the  cen- 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAVING  BRICK.  191 

ter  of  the  liquid  mass  as  shown  in  Fig.  17.  This  would  result  in  surface 
tension. 

The  practical  conclusion  from 
the  above  discussion  of  greatest 
interest  in  connection  with  plas¬ 
ticity,  is  that  when  the  surround¬ 
ing  fluid  has  the  greater  poten¬ 
tiality,  flocculation,  or  drawing 
together  of  the  solid  particles 
will  result.  When  the  solid  'par¬ 
ticles  have  the  greatest  poten¬ 
tiality,  deflocculation  or  separa¬ 
tion  of  the  solid  particles  will  re¬ 
sult.  Citations  by  the  score 
could  be  presented  showing  that 
clays  can  be  fioccula{ed,  or  de- 
fiocculated,  depending  upon  the 
FIG'  17'  DcfurgSSS£0n^  f°rces  material  carried  in  solution  by 

the  water  used  in  tempering.  It 
is  of  interest,  therefore,  to  consider  the  various  solutions  and  their  ef¬ 
fect  on  clays. 

Solutions  causing  deflocculation — Johnson  in  “How  crops  feed”  cites 
a  great  many  instances  where  solutions  of  organic  compounds  have 
caused  deflocculation  of  soils.  Ackison1  has  shown  that  tannin  will  de- 
flocculate  clay  so  thoroughly  that  when  a  thin  slip  of  clay  suspended  in 
a  solution  of  tamin  is  poured  onto  a  filter  paper  the  water  passing 
through  will  be  very  turbid.  Ammonia  is  used  in  the  water  when  a 
clay  is  being  disintegrated  preparatory  to  mechanical  analyis.  Pe¬ 
troleum  is  greedily  absorbed  by  clay  because  of  its  low  surface  tension 
or  potentiality,  being  held  between  the  minute  grains  of  clay  by  virtue 
of  the  higher  potentiality  of  the  clay  grains.  Whitney1  has  shown  that 
cotton  seed,  meal,  tankage,  etc.,  have  similar  effects. 

It  will  be  important  to  note  that  the  surface  tension  of  solutions 
which  cause  defiocculation  of  grains  of  pure  clay  substance  is  without 
exception  lower  than  the  surface  tension  of  water.  It  will  also  be  impor¬ 
tant  to  note  that  physical  differences  in  conditions  such  as  degree  of 
concentration  of  the  solution,  temperature,  etc.,  that  tend  to  decrease 
surface  tension  affect  defiocculation.  For  example,  it  is  a  common  ex¬ 
perience  of  chemists  that  boiling  for  the  purpose  of  extracting  soluble 
salts  often  so  thoroughly  deflocculates  the  clay  that  even  filtering  through 
a  Gooch  crucible  will  not  clearify  the  filtrate. 

In  the  following  tables  the  surface  tension  of  water  and  of  various 
solutions  is  given. 


1A.  c.  S.,  Vol.  VI,  p.  44. 

2  Bull.  4,  Weather  Bureau,  Dept,  of  Agr.,  p.  17. 


192 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS, 


[BULL.  NO.  9 


Table  XXII— (1.) 


The  surface  tension  of  water  and  alcohol  in  contact 
with  air. 


Temperature  C° 

Surface  tension  in  dynes  per  centimeter. 

Water. 

Ethyl  alcohol. 

0° . 

75.6 

23.5 

74.9 

23.1 

10 . . 

74.2 

22.6 

15 . 

73.5 

22.2 

20 . 

72.8 

21.7 

25 . 

72.1 

21.3 

30 . 

71.4 

20.8 

35 . 

70.7 

20.4 

40 . 

70.0 

20.0  • 

45 . 

69.3 

19.5 

50 . 

68.6 

19  1 

55 . 

67.8 

18.6 

60 . 

67.1 

18.2 

65 . 

66.4 

17.8 

70 . 

65.7 

17.3 

75 . . 

65.0 

16.9 

80 . 

64.3 

85 . 

63.6 

90 . 

62.9 

95 . 

62.2 

100 . 

61.5 

Table  XXIII  (2). 

Miscellaneous  Liquids  in  Contact  with  Air. 


Liquid. 

Temp.  C0 

Surface 

tension  in  dynes 
per  centimeter. 

Authority. 

Acetone . 

14.0 

25.6 

Average  of  various. 

Acetic  acid . 

17.0 

30.2 

.  .do . 

Amvl.  alcohol . 

15.0 

24.8 

.  .do . 

Benzine . 

15.0 

28.8 

.  .do . 

Butvric  acid . 

15.0 

28.7 

.  .do . 

Carbon  disulphide . 

20.0 

30.5 

Quincke . 

Chloroform . 

20.0 

28.3 

Average  of  various. 

Ether  . 

20.0 

18.4 

.  .do . 

Glycerine . 

17.0 

63.14 

Hall . 

Hexane  . 

0  0 

21  2 

Schiff . 

Hexane . 

68.0 

14.2 

.  .do . 

Mercury . 

20.0 

470.0 

Average  of  various. 

Methyl  alcohol . 

15.0 

24.7 

.  .do . 

Olive  oil . 

20.0 

34.7 

.  .do . 

Petroleum . 

20.0 

25.9 

Magie . 

Propyl  alcohol . 

5.8 

25.9 

Schiff . ' . 

Propyl  alcohol . 

97.1 

18.0 

.  .do . 

Tolnol . 

15.0 

29.1 

.  .do . 

Tolnol . 

109.8 

18.9 

.  .do . 

Turpentine . 

21.0 

28.5 

Average  of  various. 

1  Smithsonian  physical  tables.  Third  revised  edition,  p.  128. 
2 Smithsonian  tables,  Ibid. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


193 


Table  XXIV  (1). 


Salts  in  Solution. 

Density. 

Temp.  C° 

Surface  tension  in 
dynes  per  Cm. 

BaCl2 . 

1.2820 

15-16 

81.8 

.  .do . 

1.0497 

15-16 

77.5 

CaCl2  . 

1.3511 

19 

95.0 

1.2773 

19 

90.2 

HC1 . 

1.1190 

20 

73.6 

do  . 

1.0887 

20 

74.5 

.  .do  . 

1.0242 

20 

75.3 

KC1 .  . . 

1.1699 

15-16 

82.8 

.  .do . 

1.1011 

15-16 

80.1 

.  .do . 

1.0463 

15-16 

78.2 

MgCl2 . 

1.2338 

15-16 

90.1 

.  .do . 

1.1694 

15-16 

85.2 

.  .do . 

1.0362 

15-16 

78.0 

NaCl  . 

1.1932 

20 

85.8 

.  .do . 

1.1074 

20 

80.5 

.  .do . 

1.0360 

20 

77.6 

NH4C1  . 

1.0758 

16 

84.3 

.  .do . 

1 .0535 

16 

81.7 

.  .do . . 

1.0281 

16 

78.8 

SrCl2  . 

1.3114 

15-16 

85.6 

.  .do . 

1 . 1204 

15-16 

79.4 

.  .do . '. . . 

1.0567 

15-16 

77.8 

k2co3 . 

1.3575 

15-16 

90.9 

.  .do . 

1.1576 

15-16 

81.8 

.  .do . , . . 

1.0400 

15-16 

77.5 

Na2CU3 . 

1.1329 

14-15 

7.9.3 

.  .do . . . 

1.0605 

14-15 

77.8 

.  .do . 

-1.0283 

14-15 

77.2 

K  No, . 

1.1263 

14 

78.9 

.  .do . . ; . 

1.0466 

14 

77.6 

CuS04 . 

1.1775 

15-16 

78.6 

.  .do . 

1.0276 

15-16 

77.0 

h2so4 . 

1.8278 

15 

63.0 

.  .do . 

1.4453 

15 

79.7 

.  .do  ..*. . 

1.2636 

15 

79.7 

k2so4 .  ...  . 

1.0744 

15-16 

78.0 

.  .do . . . . . 

1.0360 

15-16 

77.4 

MgS04 . 

1.2714 

15-16 

83.2 

.  .do . 

1.0680 

15-16 

77.8 

Mn2SQ4 . 

1.1119 

15-16 

79.1 

.  .do . . . 

1.0329 

15-16 

77  3 

ZnS04 . 

1.3981 

15-16 

83.3 

.  .do . 

1.2830 

15-16 

80.7 

.  .do . 

1.1039 

15-16 

77.8 

Smithsonian  Tables  Ibid. 


Points  to  be  noted  in  table — Notes  :  1.  Solution  of  organic  compounds  have  a 
much  lower  surface  tension  than  water.  2.  Surface  tension  decreases  as  the  tem¬ 
perature  increases.  3.  As  the  density  of  the  solution  increases  surface  tension  in¬ 
creases. 

1  From  the  above  the  following  conclusions  regarding  deflocculation  ap¬ 
pears  : 

1.  It  has  been  shown  that  solutions  of  organic  compounds  cause  defloc¬ 
culation.  It  is  needless  to  go  into  further  discussion  of  this  point,  for  the 
facts  that  have  been  stated  are  well  understood  by  practical  potters  and  agri¬ 
cultural  chemists. 

2.  Increased  temperature  assists  in  producing  deflocculation.  Potters 
who-  use  hot  water  in  their  Plungers  and  brick  manufacturers  who  use  hot 
water  in  their  pug-mills  have  learned  that  clays  slake  and  develop  plasticity 
more  easily  with  hot  than  with  cold  water.  These  cases  find  their  parallel 
in  the  laboratory  when  clay  slip  is  boiled  in  the  process  of  soluble  salt  de¬ 
termination.  Deflocculation  is  increased  by  the  use  of  hot  water. 

3.  Increased  density  of  a  deflocculating  solution  does  not  increase  its 

efficiency.  “Ammonia  (1)  has  a  very  marked  action  in  breaking  up  soils 
containing  particles  less  than  0.005  mm.  in  diameter.  .  .  .  One  drop  of 


l  Bull.  No.  24,  Bureau  of  Soils,  U.  S.  Dept,  of  Agr.,  p.  22-24. 


—13  G 


194 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


ammonia  (added  to  5  grams  or  sample  in  50  cc.  of  water)  does  not  seem 
to  be  sufficient  to  break  up  the  flocculations  completely,  but  no  great  change 
is  produced  by  the  addition  of  more  than  5  drops  to  50  cc.  of  water” 

•  The  preceding  facts  given  by  our  foremost  agronomists,  when  consid- 
fered  in  the  light  of  the  fact  that  increased  concentration  of  a  solution 
increases  its  surface  tension,  are  proof  of  the  deduction  that  when  the 
potential  of  the  solid  particles  is  greater  than  that  of  the  surrounding 
fluid,  deflocculation  ensues.  In  the  case  of  the  ammonia  solution,  in¬ 
creased  concentration  by  the  addition  of  more  than  5  drops  of  ammonia 
would  so  increase  the  surface  tension  and  consequently  the  potentiality 
of  the  solution  as  the  equalize  the  potentials  of  the  soil  particles  and  the 
solution. 

Solutions  causing  flocculation — Having  discussed  the  deflocculating 
solutions  in  detail,  it  will  not  be  necessary  to  dwell  at  length  on  the 
flocculating  solutions,  for  the  effect  on  clay  of  each  class  of  solution  is 
the  converse  of  the  other.  It  is  important  to  note  that  solutions  which 
have  surface  tensions  higher  than  that  of  water  tend  to  cause  floccula¬ 
tion. 

The  nature  of  solids  affects  flocculation  in  several  ways.  First,  if  the 
clay  or  soil  under  examination  contains  a  large  quantity  of  calcium  or 
magnesium  carbonate1,  it  has  been  found  that  solutions  having  a  surface 
tension  as  low  as  that  of  ammonia  will  cause  flocculation.  Data  are  not 
available  concerning  clay  mixtures  high  in  other  minerals,  but  as  is  about 
to  be  shown,  clays  that  have  comparatively  low  content  of  clay  substance 
probably  have  as  an  average  for  the  several  mineral  grains  a  low  poten¬ 
tial  in  comparison  with  the  potential  of  water.  Clays  high  in  products 
of  decomposition  of  organic  matter  may  be  flocculated  by  ammonia.  In 
fact  the  “potential”  of  the  impure  clays  may  be  so  low  as  to  permit  am¬ 
monia  solutions  to  flocculate  their  grains.  Pure  clays,  i.  e.,  kaolins,  re¬ 
quire  for  their  flocculation  solutions  having  a  “potential”  that  is  higher 
than  that  of  pure  water. 

Second,  near1  the  surface  of  any  soil  there  is  a  concentration  of  solu¬ 
tions.  This  is  adsorption.  If2  the  solid  is  exceedingly  porous,  this  ten¬ 
dency  to  concentration  near  the  surface  is  heightened.  It  is  well  known 
that  salts,  which  are  concentrated  near  the  surface  of  solids  are  precipi¬ 
tated  or  at  least  are  left  clinging  to  the  solids  when  the  water  is  with¬ 
drawn.  Soils3 4,  even  sand,  possess  the  property  of  attracting  and  fully 
absorbing  salts  which  cannot  be  wholly  washed  out  by  new  quantities  of 
water.  Solutions  of  many  of  the  salts  are  materially  weakened  when 
brought  in  contact  with  solids,  because  of  the  adsorption  of  the  salts,  but 
if  the  surface  of  the  solid  be  relatively  small  no  weakening  of  the  solu¬ 
tion  may  be  perceptible. 

Summary — The  well-known  facts  concerning  a  plastic  clay  when 
wetted  with  water  are,  first,  that  its  finer  portions  are  composed  of  a 
countless  number  of  minute  grains,  the  composition  of  which  has  been 
shown  to  agree  closely  with  that  of  pure  clay  substance ;  second,  that  even 
the  coarser  grains  are  composed  largely  of  kaolinite  and  other  minerals 

1  U.  S.  Dept,  of  Agr.,  Bureau  of  Soils,  Bull.  24,  p.  24. 

2  U.  S.  Dept.  Agr.,  Rept.  No.  64,  p.  142. 

3  Comp.  Johnson,  How  Crops  Feed.  p.  173. 

4  Comp.  Johnson,  How  Crops  Feed,  p.  334. 


PURDY]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  195 

cemented  into  clusters  or  bundles;  third,  that  clays  having  a  high  con¬ 
tent  of  minerals  other  than  kaolin,  are  flocculated  by  solutions  having 
a  surface  tension  lower  than  that  of  water,  while  the  clays  which  are 
practically  pure  kaolinite  in  composition  require  for  their  flocculation 
solutions  that  have  a  surface  tension  higher  than  water ;  fourth,  that  clay 
particles  extract  salts  from  solutions  and  hold  them  near  and  on  their 
surface  at  a  high  degree  of  concentration;  fifth,  that  clay  substance  ex¬ 
hibits  this  property  of  adsorbing  salts  to  a  much  higher  degree  than  any 
of  the  common  anhydrous  minerals,  a  fact  that  makes  the  extreme  fine¬ 
ness  of  the  "clay  substance  in  clays”  of  considerable  significance. 

The  known  facts  concerning  solutions  are :  first,  that  all  solutions  have 
a  surface  tension  which  is  increased  with  increased  concentration;  sec¬ 
ond,  that  those  solutions  which  have  a  surface  tension  higher  than  that  of 
pure  water,  tend  to  cause  flocculation  of  kaolin  grains. 

Oh  putting  together  the  known  facts  concerning  clay  and  water,  it  is 
evident  that  the  film  of  water  surrounding  the  grains  of  clay ,  ( when  the 
mass  is  in  a  plastic  condition)  has  a  very  high  potential ,  owing  to  the 
high  degree  Of  concentration  of  the  salts  that  are  held  to  the  kaolin  grains 
by  adsorption. 

SUPPOSED  HISTORY  OF  THE  DEVELOPMENT  OF  PLASTICITY  OF  CLAYS  IN 

NATURE. 

Daubree1,  Cushman2  and  Mellor3  have  disintegrated  feldspar  in  water 
either  by  grinding  or  by  boiling.  In  all  cases,  the  liquid  in  which  the 
feldspar  was  ground  contained  alkali  in  solution.  Mellor  found  that  not 
only  did  the  solution  give  alkaline  reactions,  but  the  <(  outlines  (of  the 
solid  particles)  could  be  more  readily  stained  ivith  saff ranine  or  with 
malachite  green  than  before  the  action” 

Since  the  larger  part  of  the  clay  substance  is  derived  from  the  dis¬ 
integration  of  feldspar,  it  can  be  considered  that  there  was  formed  at 
the  time  of  "kaolinization”  insoluble  hydrous  silicate,  of  alumina,  soluble 
potash  salt  and  soluble  silicic  acid.  If  feldspar  has  been  disintegrated 
by  atmospheric  agencies,  water  and  carbonic  acid,  and  the  residual  mass 
is  so  situated  as  not  to  allow  the  soluble  salts  to  be  washed  away,  they 
will  be  retained  in  part  by  adsorption  and  in  part  by  recombination, 
forming  zeolitic  masses.  Data  are  not  available  to  warrant  the  state¬ 
ment  that  plastic  kaolins  formed  in  situ  owe  their  plasticity  to  these  ad¬ 
sorbed  salts,  or  that  they  even  contain  adsorbed  salts.  We  do  know, 
however^  that  nearly  all  kaolins  contain  alkalies  that  can  not  be  shown 
to  be  present  as  constituents  of  such  minerals  as  feldspar  or  mica.  Fur¬ 
ther  we  know  that  "the  less4  free  alkali  a  clay  contains  the  more  will  it 
adsorb.”  We  know  also  that  clays  which  have  been  formed  from  feldspar 
under  the  disintegrating  influence  of  fluorine  are  not  plastic,  and  con^ 
tain  fluorspar5  and  other  fluorine  compounds. 

1  Am.  Rep.  Smithsonian  Inst.,  1862,  228. 

2  U.  S.  Dept.  Agr.,  Bull.  No.  92. 

3  Eng.  Cer.  Soc.,  Vol.  pt.  1,  p.  72. 

4C.  F.  Binns.  A.  C.  S.,  Vol.  VIII,  p.  206. 

5  Jackson  and  Richardson,  Eng.  Cer.  Soc.,  Vol.  II,  p.  59.  (1903-4.) 


196 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


Cushman1  reports  that  the  residue  left  after  disintegrating  the  feldspar 
and  washing  out  the  portions  which  have  been  rendered  soluble,  is  com¬ 
posed  of  very  minute  particles.  If  this  insoluble  portion,  kaolin,  had 
been  formed  by  nature,  in  whose  laboratory  reactions,  precipitations,  etc., 
extend  over  an  almost  infinitely  longer  time  than  is  given  to  similar 
physical-chemical  phenomena  in  the  laboratory.,  there  is  no  doubt  but 
that  these  minute  particles  of  kaolin  would  arrange  themselves  in  the 
thin  plate-like  clusters  that  are  characteristic  of  this  substance,  just  as 
did  the  Leasburg  clay  cited  by  Wheeler.  Conditions  will  control  the 
size  of  the  plate-like  crystals  so  developed.  In  many  kaolins  these  plate 
forms  are  discernable,  ranging  from  those  of  sub-microscopic  dimensions 
up  to  those  that  can  be  readily  measured  in  the  microscope.  In  a  clay 
examined  by  the  writer  not  only  Avere  there  crystals  of  measureable  size, 
but  they  appeared  to  be  compounded,  i.  e.,  made  up  of  several  crystals, 
which  could  not  be  separated  to  an  appreciable  extent  by  vigorous  shak¬ 
ing  in  distilled  water.  Under  natural  conditions,  therefore,  where  the 
disintegrating  water  readily  runs  or  seeps  away,  carrying  the  soluble  por¬ 
tions  and  leaving  the  insoluble  “residual  clay”  in  situ ,  we  can  expect  to 
find  a  deposit  that  is  more  or  less  crystalline,  depending  upon  the  attend¬ 
ing  conditions. 

These  deposits,  we  know,  are  practically  non-plastic.  We  know  fur¬ 
ther  from  Ackison's  experiments  and  the  testimony  of  many  agricultural 
chemists,  that  these  grains  can  be  deflocculated  by  organic  solutions. 
Since  surface  waters  generally  contain  organic  substances  in  solution, 
and  since  proximity  of  vegetable  growth  can  give  rise  to  a  deposit  of 
decaying  vegetable  matter  on  kaolin  beds,  it  is  easy  to  see  how  such  a  de¬ 
flocculation  can  take  place  in  situ ,  and  especially  so  if  the  clay  be  moved 
by  running  water  and  deposited  in  the  lower  lands.  By  virtue  of  this 
deflocculation  the  clay  has  a  smoother  feel,  i.  e.,  texture,  and  thereby 
assumes  a  pseudo-plasticity.  This  fact  has.  given  rise  to  the  fineness  of 
grain  theory  of*  plasticity. 

These  deflocculated  particles  of  kaolin  have,  as  has  been  shown,  a 
high  adsorptive  power.  Whatever  salts  may  be  in  solution  in  the  passing 
waters,  or  may  be  carried  upward  from  lower  strata  by  rising  waters, 
will  be  adsorbed  by  the  kaolin  particles.  Now,  depending  upon  the  de¬ 
gree  of  deflocculation,  amount  of  adsorption,  and  the  kind  of  salt  so  ad¬ 
sorbed,  plasticity  will  be  developed. 

When  non-plastic  kaolins  are  wetted  with  water,  they  are  compressed 
into  shapes  only  with  difficulty,  and  when  dried  they  either  fall  to  pieces, 
as  would  so  much  fine  sand,  or  have  so  weak  a  bond  that  they  are  easily 
crumbled.  The  finer  the  particles,  as  with  the  case  of  sand,  the  more 
readily  can  they  be  shaped  into  pieces  that  will  retain  their  form,  but  no 
matter  how  finely  sub-divided  the  grains  may  be,  the  mass  is  still  very 
friable.  In  this  fine  condition  the  kaolin  no  doubt  possesses  every  chem¬ 
ical  and  physical  property  possessed  by  the  plastic  kaolin  (ball  clay) 
save  that  of  plasticity.  It  has  water  chemically  combined,  molecular 
attraction,  and  adsorbing  properties.  It  'becomes  plastic  only  when  it 


1U.  S.  Dept.  Agr,  Bull.  No.  92. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


197 


has  adsorbed  salts,  the  solution  of  which  exhibits  a  high  surface  tension , 
or  as  Whitney  would  express  it,  which  have  a  relatively  high  potentiality . 
Clays  having  adsorbed  salts  and  consequently,  plasticity  are  no  longer 
friable  when  molded  but,  on  the  contrary,  they  are  exceedingly  hard. 

It  is  because  of  this  adsorption  property  which  in  kaoline  grains 
seems  to  be  manifested  to  a  higher  degree  than  in  any  other  mineral 
substance,  with  perhaps  the  exception  of  zeolites,  that  many  find  reason 
to  believe  that  plasticity  is  due  to  a  pectoidal  structure  of  the  kaolin 
grains.  Since,  however,  they  cannot  show  that  those  substances  which 
are  known  to  have  a  pectoidal  or  colloidal  structure  can  be  made  to 
show  or  develop  plasticity,  and  since  colloids  cannot  be  extracted  from 
plastic  clays,  rendering  them  non-plastic,  nor  added  to  non-plastic 
kaolins  rendering  them  plastic,  we  must  conclude  that  this  theory  is 
hardly  tenable. 

To  what  this  great  adsorptive  power  of  clays  is  due  has  not  as  yet  been 
determined.  We,  however,  must  accept  the  existence  of  this  property 
as  a  proved  fact.  We  must  also  concede  that  when  water  is  added  to  a 
clay,  that  portion  which  envelops  the  very  minute  solid  particles  having 
a  relatively  large  surface  area  in  proportion  to  their  volume,  and  hold¬ 
ing  salts  by  absorption,  will  be  highly  concentrated,  that  the  potential 
of  this  film  will  he  very  much  more  than  those  portions  of  the  water 
farthest  away  from  the  solid  particles;  and  finally,  as  shown  by  the 
flocculation  of  the  clay  particles,  the  potential  of  this  saturated  film 
of  water  is  higher  than  the  potential  of  the  kaolin  particles. 

The  writer  bases  his  assumptions  as  to  the  cause  of  plasticity  on 
known  facts :  Adsorption  of  salts  by  the  kaolin  grains  and  the  con¬ 
sequent  high  potential  of  the  water  film  which  surrounds  the  grains  when 
a  clay  mass  is  in  a  plastic  condition.  On  these  assumptions,  the  cause 
of  the  latent  plasticity  when  clay  is  dry  and  the  developed  plasticity 
when  it  is  wet,  are  obvious. 

Fineness  of  grain,  molecular  attraction,  adsorptive  property,  are 
conditions  that  permit  of  the  adsorption  of  salts.  In  other  words,  they 
are  necessary  conditions. 

METHODS  OF  MEASURING  PLASTICITY. 

General — There  have  been  many  methods  devised  for  measuring 
placticity.  The  methods  suggested  by  Zischokke1 2  and  Grout3  seem  to 
be  the  most  rational  of  any,  for  in  them  the  resistance  to  deformation 
and  amount  of  flow  before  rupture,  two  characteristic  properties  of 
plastic  bodies,  are  measured.  These  methods  are  based  on  the  same 
principle  as  the  well-known  but  crude  method  of  testing  plasticity  by 
squeezing  a  ball  of  plastic  clay  between  the  tips  of  the  forefinger  and 
thumb,  and  making  a  mental  note  of  the  amount  of  pressure  required 
to  affect  a  given  degree  of  deformation. 

The  test  developed  by  this  Survey  involves  the  tensile  strength  of 
the  plastic  mass  rather  than  its  resistance  to  compression,  as  in  the 

1  For  description  of  these  methods  see  “Clays ;  Occurrence,  Properties  and  Uses” 
by  H.  Ries.  Wiley  .and  Sons,  1906. 

2  Thon-Industrie - Zeitung,  No.  120,  p.  1658.  (1905.) 

3W.  Va.  Geol.  Surv.,  III.  p.  40. 


198 


PAVING  BEICK  AND  PAVING  BEICK  CLAYS. 


[BULL.  NO.  9 


Zschokke  and  Grout  methods.  It  is  believed  that  a  tensile  test  gives 
a  more  accurate  rating  of  the  tenacity  with  which  the  several  grains 
cling  to  one  another,  for  in  this,  friction  between  the  non-plastic  grains 
and  interference  to  flowage  by  the  larger  ones  crowding  into  one 
another  is  entirely  eliminated. 

Shape  of  the  test  piece — The  special  features  of  the  shape  and  size 
of  the  briquette  employed  in  this  test  are,  first,  narrow  neck,  (%"), 
wide  ends  ( — ")  and  straight  sides  to  fit  the  jaws,  as  explained  later. 
The  smallest  portion  is  cubical  in  shape,  being  %"  x  %"  x 

The  clips — In  previous  experiments  it  was  learned  that  the  Standard 
Fairbanks  clips,  using  the  standard  shape  and  size  of  briquette,  would 
permit  the  stretching  of  the  briquettes  until  they  would  slip  out  of  the 
jaws.  Special  clips  were  therefore  made  to  fit  the  briquette.  These 
clips  were  designed  after  Orton1,  differing  from  his  only  in  dimensions, 
angle  of  nip  between  the  jaws,  and  manner  of  adjustment. 

Manufacture  of  briquettes — Clay  was  mixed  to  a  thick  slip,  cast  and 
cut  into  briquettes  by  the  Fox  method,  as  described  under  tensile 
strength.  When  the  cast  slab  had,  in  the  opinion  of  the  operator,  as¬ 
sumed  its  maximum  plasticity,  the  briquettes  were  cut  an.d  forced  into 
steel  molds  under  a  constant  pressure  of  fifty  pounds.  This  weight  was 
applied  slowly  but  the  briquette  was  not  allowed  to  remain  under  pres¬ 
sure  after  it  had  received  its  full  load. 

Adjustment  and  calibration  of  the  machines — Before  the  Fairbanks 
machine  could  be  used,  the  balance  beam  had  to  be  poised  to  allow  for 
the  difference  between  the  standard  and  our  special  clips. 

For  measuring  the  stretch  which  the  briquettes  suffered,  the  small 
adjusting  wheel  was  calibrated  so  that  the  peripheral  distance  through 
which  the  wheel  was  turned  would  represent  the  distance  the  under  clip 
had  been  lowered.  The  amount  of  stretch  which  the  briquettes  suf¬ 
fered  at  any  time  during  the  test  was  measured  by  the  fractional  num¬ 
ber  of  turns  of  the  adjusting  wheel  required  to  lower  the  under  jaw 
sufficiently  to  keep  the  beam  in  a  predetermined  position. 

Method  of  procedure — The  plastic  briquette  was  carefully  placed  in 
the  clips  and  the  jaws  adjusted  to  it,  care  being  taken  to  see  that  the 
jaws  on  either  side  were  at  the  same  angle.  The  lower  clips,  suspended 
by  counterpoise,  were  kept  in  a  vertical  line  by  hand  guidance.  Very 
small  shot  was  allowed  to  run  into  the  pail  slowly  until  a  rupture  oc- 
cured  at  the  neck  of  the  briquette.  As  soon  as  a  rupture  occurred,  the 
beam  dropped  with  a  suddenness  that  shut  off  the  flow  of  shot.  At- 
the  moment  of  rupture  the  amount  of  initial  stretch  was  noted  by  the 
fractional  number  of  revolutions  through  which  the  adjusting  wheel 
had  been  turned.  Before  removing  the  “load”  the  adjusting  wheel  was 
slowly  turned  until  the  briquettes  was  completely  torn  apart.  The  sec¬ 
ond  peripheral  distance .  through  which  the  wheel  had  been  turned  was 
noted  as  “final  stretch.” 

The  weight  of  the  shot  required  to 'cause  rupture  was  obtained  on  a 
balance  that  is  accurate  to  one  centigram.  While  the  weight  thus  ob¬ 
tained  is  not  the  force  that  was  required  to  cause  rupture,  it  does  bear  a 


lAmer.  Cer.  Soc.,  Vol.  VI,  p. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK, 


199 


constant  ratio  to  that  force.  The  shot  was  not  weighed  on  the  Fair¬ 
banks  machine  because  it  was  not  sufficiently  sensitive. 

Plasticity  modulus — It  is  obvious  that  since  all  three  factors,  initial 
stretch,  final  stretch,  and  load  required  to  cause  rupture,  must  be  consid¬ 
ered  as  being  affected  by  the  degree  of  plasticity  of  the  clay,  a  modulus 
must  be  devised  that  includes  all  three  factors.  The  one  used  in  our 
tests  was  constructed  as  follows : 

The  central  portion  of  the  briquette  is  a  perfect  cube  %"x3/4"x%" . 
On  the  assumption  that  the  volume  of  this  portion  of  the  briquette  re¬ 
mains  constant  throughout  the  test1,  and  that  its  cross  section  decreases 
proportionally  as  the  length  increases,  the  decrease  in  cross  section  in 

f  's\  1.93  ^  ^ 


centimeters  due  to  the  initial  stretch  would  be(  1.92x - .lor  - 

V  1.9+a/  1.9+a 

where  a  is  the  initial  stretch.  The  decrease  in  cross  section  after  the* 
final  stretch  (here  it  is  figured  as  though  there  has  been  no  rupture) 
x  1.9 3  1.93  \ 


would  be  equal  to 


1.9+a 


(~) 


1.9+a+b 


or,  by  reduction, 


.  1.93b 


where  b  is  the  final  stretch. 


1.92+3.8a+1.9b+a2+ab 

Now  a  measure  of  the  tension  that  is  holding  the  grains  together 
would  be  directly  proportional  to  the  load  and  inversely  proportional  to 
the  decrease  in  cross  section  of  the  briquette  due  to  stretching.  The 
modulus  must,  therefore,-  represent  a  value  that  is  directly  as  the  load 
and  inversely  as  the  product  of  the  decreases  in  cross  section  due  to  the 
initial  and  final  stretch.  Performing  this  calculation  and  collecting; 
terms  the  following  plasticity  modulus  is  obtained : 

1  (6.859  +  10.83  a  +  3.61  b  +  5.7  a2  +  3.8  ab  +  as  +  a2b)=M 

24776b 


in  which  L  =  Ioad  in  centigrams,  a  the  initial  stand,  b  the  final  stretch. 

While  the  modulus  is  very  formidable  looking  it  was  found  that  the 
test  could  be  made  and  the  plasticity  factor  calculated  quite  readily.  In 
fact  the  entire  test  required  less  time  than  did  the  Grout  test  as  carried 
out  in  our  laboratories. 

With  the  heavy  and  far  from  delicate  Fairbanks  machine  and  the 
clumsy  clips,  plasticity  factors  were  obtained  that  varied  for  any  one  clay 
not  more  than  50  per  cent  and  in  some  cases  only  13  per  cent  on  six: 
briquettes.  This  percentage  of  variation  was  considered  too  high  to  at¬ 
tach  any  value  to  the  obtained  data,  and  they  are,  therefore,  not  reported. 
It  is  believed  that  with  a  more  delicate  apparatus  this  method  of  measur¬ 
ing  plasticity  would  give  very  close  results  and  .that  the  data  obtained 
would  be  a  true  measure  of  plasticity. 


1  This  is  no  doubt  an  incorrect  assumption.  In  iron  they  figure  that  the  length 
increases  four  times  as  much  as  the  cross  section  decreases,  in  other  words  that 
in  the  stretch  the  volume  of  the  test  piece  actually  increases. 

2  The  decrease  in  cross  section  of  the  briquette  is  calculated  instead  of  taking 
the  observed  increase  in  length  because  it  and  the  bond  or  strength  of  the  mass- 

are  directly  proportional  while  the  length  is  not. 


200 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[bull.  no.  9 


CHEMICAL  PROPERTIES. 

Value  of  Chemical  Analyses. 

Because,  in  private  correspondence  and  on  every  public  and  semi-public  oc¬ 
casion  that  has  afforded  opportunity,  the  writer  has  taken  a  stand  against  the  large 
absolute  value  and  importance  of  chemical  analysis  of  clays  that  is  contrary  to 
views  popularly  held  by  practical  clay  workers  and  encouraged  by  manv  of  the 
State  Survey  reports,  the  discussion  of  the  chemical  properties  of  clays  is  intro¬ 
duced  by  liberal  quotations  from  the  most  eminent  of  ceramic  chemists.  Dr.  Her¬ 
mann  Seger. 

“Thei  demands  which  the  cement  and  the  ceramic  industries  make  on 
the  qualities  of  clay  are  as  different  as  the  purposes  which  these  industries 
pursue. 

“In  the  manufacture  of  Portland  cement  we  have  in  mind  the  obtaining 
of  a  product  of  a  definite  chemical  composition,  and,  since  the  character  of 
clay  as  such  must  completely  vanish  in  this,  the  mutual  relation  of  the  indi¬ 
vidual  constituents  is  to  he  considered  above  all  things,  and  the  physical 
condition  in  which  these  are  found  be  considered  only  as  far  as  it  opposes 
greater  or  less  difficulties  to  the  destruction  of  the  clayey  character. 

“The  clay  industries,  on  the  other  hand,  pursue  a  quite  different  purpose 
in  the  treatment  of  their  raw  material.  The  limits  within  which  the  chem¬ 
ical  constitution  of  clay  may  vary  are  very  wide,  and,  since  the  clayey  char¬ 
acter  of  the  material  is  to  be  preserved,  its  physical  qualities  and  those  of 
its  essential  and  accessory  constituents  are  to  be  placed  in  the  foreground 
While  for  such  a  purpose,  the  chemical  composition  of  clay,  as  a  whole,  ap¬ 
pears  more  indifferent  and  accidental,  inasmuch  as  it  depends  on  the  mutual 
relation  of  clay,  rock  flour,  sand,  accidental  admixtures  and  their  chemical 
constitution,  the  physical  properties  of  the  same,  the  grain  and  its  form,  cap¬ 
illarity,  plasticity,  fusibility,  etc.,  are  of  greater  importance,  and  the  chemi¬ 
cal  constitution  of  each  of  these  constituents  is  to  be  considered  only  as  far 
as  it  permits  us  to  infer  the  physical  properties  of  the  whole.  II  is  surely 
a  serious  mistake  to  treat  material  so  heterogeneous  chemically  and  me¬ 
chanically,  as  the  clays  and  earths  used  in  the  ceramic  industries,  like  sub¬ 
stances  chemically  and  physically  homogeneous,  as  for  example  glass,  and  to 
base  conclusions  with  regard  to  their  properties  on  their  chemical  composi¬ 
tion. 

“The  chemical  changes  which  the  materials  of  the  ceramic  industries 
suffer  in  the  course  of  manufacture,  step  into  the  background,  with  the 
exception  of  the  loss  of  chemically  bound  water,  which  has  as  a  consequence 
the  loss  of  plasticity,  and  must  not  be  produced  in  the  same  degree  as  in 
the  manufacture  of  cement  and  glass,  or  the  material  will  lose  its  earthy 
character.  In  fact,  it  seems  advisable  to  drop  the  investigation  of  the  chem¬ 
ical  composition  of  clay  as  a  whole,  and  put  in  its  place  a  deeper  study  of 
the  composition  of  the  essential  and  accidental  constituents,  in  order  to 
to  infer  the  properties  of  the  whole  from  the  properties  of  the  compounds 
thus  obtained.  For  example,  we  need  not  ask  how  much  pure  clay  and 
silicic  acid  we  have  in  clay,  but  what  part  of  the  clay  and  silicic  acid  be¬ 
longs  to  the  sandy  constituents,  what  part  to  the  silty,  or  the  clayey  con¬ 
stituent,  and  what  physical  properties  must  we,  according  to  these  data, 
ascribe  to  the  sand,  rock  dust,  clay,  etc.,  individually. 

“It  cannot  be  denied  that  in  the  examinations  of  clays  scrupulously  accur¬ 
ate  analyses  of  the  material  have  heretofore  been  made,  but  that  little  has 
been  learned  concerning  structure,  condition  of  plasticity,  power  of  absorbing 
water,  shrinkage  on  drying  and  burning,  form  and  size  of  grains  of  sand, 
and  rock  dust,  concerning  the  pecularities  of  the  concretions,  and  concern¬ 
ing  efflorescences  and  incrustations.  In  the  consideration  of  the  properties 
of  clay  for  the  purposes  of  the  clay  industries  we  must  put  ourselves  more 
upon  a  physical  than  a  chemical  standpoint. 


l  Dr.  Hermann  Segar,  the  Collected  Writings  of,  Trans,  by  A.  C.  S.  p.  8-11. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK. 


201 


“Ifi  chemical  analysis  has  discovered  a  fixed  relation  between  alumina, 
silicic  acid  and  flux,  we  know  that  these  constituents  belong  essentially  to 
a  single  well-characterized  combination,  so  that  we  can  take  the  degree  of 
refractioriness  from  the  laws  established  by  Bischofand  Ritchers  with  a 
greater  or  less  assurance,  .according  as  this  substance  is  present  in  a  greater 
or  less  degree  of  purity.  However,  if  we  should  proceed  in  a  similar  way 
in  the  investigation  of  brick  clays,  we  would  get  a  theoretical  result  so 
very  different  from  the  practical  results  that  it  would  have  no  value  what¬ 
ever  in  regard  to  the  knowledge  of  the  material.  The  chemical  analysis 
gives  us  only  an  average  of  the  composition  of  the  components  forming  the 
clay,  differing  very  widely  in  their  chemical  composition  and  their  physi¬ 
cal  properties.  Since  the  clay,  after  burning,  preserves  its  earthy  charac¬ 
ter,  and  the  various  constituents  act  only  superficially  on  each  other,  the 
chemical  analysis  gives  us  absolutely  no  clue  for  the  deduction  of  definite 
properties  of  the  whole. 

“ Two  brick  clays  may  have  exactly  the  same  composition  and  still  differ 
in  every  respect,  because  the  complete  analysis,  for  example,  gives  us  no 
idea  whatever  as  to  how  much  silicic  acid,  alumina  and  flux  belong  to  the 
clay  substance,  to  the  rock  dust,  and  to  the  sand  individually;  for  instance, 
in  the  one  case  all  or  the  greater  part  of  the  flux  may  belong  to  the  clay 
substance,  in  another  case  to  the  constituents  which  make  clays  lean,  and 
accordingly  the  properties  of  the  compound  be  subject  to  the  greatest  varia¬ 
tion  with  the  same  percentage  composition.  In  the  one  case  it  may  be  the 
clay,  in  another  the  rock  dust  or  sand,  which,  with  the  same  percentage 
composition  of  the  whole,  is  the  most  fusible  constituent,  as  admixed  iron 
oxide  or  carbonate  of  lime,  which  according  to  the  manner  of  distribution 
are  inclined  to  have  the  strongest  effect  sometimes  on  the  clay,  sometimes 
on  the  rock  dust  and  sand,  and  thereby  produce  a  number  of  variations 
which  find  not  the  slighest  explanation  by  a  simple  chemical  analysis. 

“If  we  conclude  from  this  that  chemical  analysis  can  claim  only  a  limited 
value  for  the-  discovery  of  the  properties  of  brick  clay,  such  a  judgment 
would  be  highly  one-sided  and  inaccurate.  .  .  .  For  our  purpose  it  is 

especially  the  physical  properties  of  clay  that  are  of  greatest  importance  in 
judging  the  same,  and  the  chemical  properties  only  as  far  as  they  supple¬ 
ment  the  former.  Here,  therefore,  to  express  it  in  a  few  words,  it  will  be 
the  task  of  the  chemical  analyst  to  determine  the  composition  of  the  con¬ 
stituents  that  are  physically  alike,  that  of  the  clay,  rock  dust,  sand,  and 
accessory  constituents,  separately  and  singly,  and  to  make  possible  a  com¬ 
parison  of  these  with  each  other.  In  this  way  we  are  able  to  get  a  good 
idea  of  the  properties  of  the  components,!  whereas  an  analysis  of  the  whole 
mass  would  be  of  little  use.  We  are  thus  convinced  of  the  necessity  of 
physical  analysis  of  clay  simultaneously  with,  or  rather  before  the  chemical, 
as  far  as  the  investigation  is  made  for  the  purposes  of  pottery  ware,  and 
especially  for  the  manufacture  of  bricks.  Even  though  scientific  men  have 
repeatedly  referred  to  the  importance  of  the  mechanical  and  physical  inves¬ 
tigations,  this  direction  of  the  investigation  has  not  been  pursued  with  such 
vigor  that  the  results  obtained  from  it  show  any  real  use  for  the  brick 
industry.” 

In  the  foregoing  statements  Dr.  Seger  has  very  forcibly  set  forth  the 
same  doctrines  that  the  writer  has  come  to  thoroughly  believe  as  a  re¬ 
sult  of  observations  in  the  factory  and  laboratory.  In  subsequent  writ¬ 
ings  Dr.  Seger  set  forth  the  value  of  what  is  known  as  the  “Rational 
..analysis”  in  which  “clay  substance,”  feldspar  and  free  silica  are  differ¬ 
entiated.  He  cited  many  cases  in  which,  with  the  aid  of  the  Rational 
analysis,  he  was  able  to  obtain  more  clearly  an  idea  of  the  constitution 

1  Seger,  loc.  cit.,  p.  36. 

2  Italics  not  in  the  original. 

3  Italics  not  in  the  original. 

4  Italics  not  in  the  original. 


202 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


and  properties  of  clay  than  he  conld  from  any  other  method  of  analysis. 
In  this,  however,  he  was  no  doubt  over  zealous,  for  later  studies  by  other 
chemists  proved  that  not  only  does  the  “Rational  analysis”  fail  to 
sharply  differentiate  between  the  “clay  substance,”  feldspar  and  free 
silica,  but  that  the  analysis  is  of  value  only  in  the  purest  clays,  viz. : 
China  and  ball  clays.  The  writer  has  made  ‘rational”  t  analysis  of  many 
types  of  clays,  and,  barring  those  used  in  the  vitreous  pottery  wares,  he 
is  compelled  to  state  that  not  once  has  he  been  able  to  obtain  a  clearer 
insight  into  the  actual  constitution  of  the  clays  than  he  could  from  the 
gross  or  ultimate  analyses.  Predictions  concerning  a-  cla}^  based  on  a 
rational  analysis  in  the  great  majority  of  cases,  go  very  far  wrong. 
After  considerable  pains  and  labor  in  the  execution  of  the  analysis  the 
operator  is  compelled  to  make  guesses  that  are  much  less  scientific  and 
accurate  than  he  would  if  he  had  merely  burned  a  piece  of  the  same  clay 
in  a  small  muffle  furnace,  and  noted  the  rate  of  change  in  color  and  dem- 
sity  with  increasing  intensity  of  heat  treatment,  a  test  that  ought  not 
to  require  more  than  three  hours  time,  and  can  be  made  by  any  one  who 
has  access  to  a  kiln. 

It  was  shown  in  a  preliminary  report  on  the  fire  clays  of  Illinois1 
that  fire  clays  having  the  same  ultimate  chemical  composition  behave 
very  differently  in  burning.  Indeed  the  chemical  composition  and  ulti¬ 
mate  fusion  period  were  very  often  found  to  coincide  in  clays  which, 
on  the  one  hand,  would  remain  open  and  porous  through  sufficiently 
long  and  severe  heat  treatments  to  make  them  fit  for  use  in  fire  brick, 
or,  on  the  other,  would  be  nearly  vitrified  under  the  heat  treatment  used 
in  burning  stoneware  and  sewer  pipe.  Such  phenomena  are  discussed 
and  illustrated  in  this  report  under  the  title  of  pyro-physical  and  chem¬ 
ical  properties  of  clays. 

In  the  manufacture  of  vitreous  floor  tile  the  writer  learned  by  prac¬ 
tical  experience  that  particular  effects  either  in  color,  vitreousness,  ulti¬ 
mate  fusibility,  or  any  other  physical  property  requisite  in  the  produc¬ 
tion  of  floor  tile,  could  not  be  duplicated  on  the  basis  of  chemical  com¬ 
position. 

This  was  also  found  true  in  the  manufacture  of  porous  white  ware 
bodies,  such  as  are  used  in  jardinieres  and  art  wares,  and  no  doubt  is  en¬ 
tertained  but  that  the  same  would  be  found  to  hold  true  to  a  large  ex¬ 
tent  in  the  manufacture  of  vitreous  china.  In  these  cases,  however, 
rational  analysis,  i.  e.,  the  determination  of  the  proportional  quantity 
of  clay  substance,  feldspar,  and  free  silica  finds  value  in  that  these  sev¬ 
eral  minerals  have  decided  effect  on  the  expansion  and  contraction  of 
the  blended  pottery  body,  and,  consequently,  upon  the  proper  fitting 
on  it  of  a  glassy  coating  (the  glaze.) 

The  only  instance  in  which  chemical  analysis  is  of  positive  aid,  aside 
from  the  explaining  of  some  observed  phenomenon,  is  in  the  execution 
and  study  of  a  systematically  planned  series  of  experiments.  Seger’s 
classical  studies  that  resulted  in  the  invention  of  the  pyrometric  cones 
would  probably  never  have  been  carried  out  had  he  not  followed  closely 

1  Purdy  and  De  Wolf,  Preliminary  Report  on  the  Fire  Clays  of  Illinois,  State 
Geol.  Surv.  of  Ill.  Year-book  1906,  pp.  137. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK. 


203 


the  chemical  analyses  of  the  raw  materials  and  planned  his  series  on 
chemical  formulae.  Following  him,  there  has  been  much  of  exceedingly 
great  value  resulting  from  researches  in  pottery  mixtures  that  would 
have  been  impossible  on  any  other  than  a  strictly  chemical  basis.  In  the 
study  of  paving  brick  clays  here  reported  the  fact  has  been  discovered 
that  the  best  paving  clays  contain  a  relatively  high  content  of  magnesia. 
Such  a  discovery  has  been  and  would  have  been  impossible  from  an 
analysis  of  an  isolated  sample.  Further,  this  fact  would  not  have  been 
noted  had  no  systematic  researches  on  a  chemical  basis  been  made  with 
pure  clays,  minerals,  and  magnesia  compounds,  showing  that  mixtures 
containing  a  comparatively  high  content  of  magnesia  have  a  long  fusion 
range,  for,  as  will  be  seen  later,  the  value  of  clays  for  paving  brick  man¬ 
ufacture,  or  even  their  fusion  range,  do  pot  always  .correlate  with  high 
magnesia  content. 

The  suggestion  made  by  Dr.  Seger  in  the  paragraphs  quoted  in  the 
introduction  to  this  chapter,  that  a  chemical  analysis  of  the  several  sub¬ 
divisions  of  the  particles  according  to  size  would  be  of  value,  is  possibly 
true.  In  fact  it  is  obvious  why  such  should  be  the  case.  The  time  and 
trouble  involved  in  making  a  thorough  mechanical  analysis  of  a  clay  into 
several  groups  having  different  ranges  in  size  of  particles  in  quantities 
sufficient  to  make  accurate  and  especially  duplicated  analyses  of  each 
group,  places  such  a  determination  out  of  consideration  as  a  commercial 
test  on  clays.  But  for  a  scientific  purpose  it  is  believed  that  the  re¬ 
sults  obtained  would  justify  the  expense  and  trouble  involved.  Such  a 
study  was  made  by  Grout  on  a  composite  sample  of  West  Virginia  clays. 
His  results  are  cited  and  discussed  on  pages  179  to  180  of  this  report. 
So  far  as  the  writer  is  aware,  Grout  was  the  first  to  make  such  an  analy¬ 
sis,  and  it  is  hoped  that  the  deductions  drawn  from  his  results  show 
justification  for  the  making  of  similar  studies  on  single  clays. 

Notwithstanding  the  fact  that  up  to  date  it  would  be  but  a  matter 
of  chance  that  an  interpretation  of  the  results  of  a  chemical  analysis 
would  agree  with  the  observed  working  properties  of  a  clay,  it  should 
not  be  concluded  that  our  chemists  may  not  in  the  near  future  devise 
a  method  of  analysis  that  will  meet  the  requirements  of  the  case.  In 
fact,  so  strongly  do  many  believe  that  this  will  come  to  pass,  that  they 
see  justification  in  making  and  reporting  chemical  analyses  of  clays  by 
geological  surveys,  as  has  been  their  wont  in  the  past.  The  writer  firmly 
believes,  however,  that  as  a  forerunner  to  such  an  event,  many  carefully 
executed  and  systematic  physical  and  chemical  researches  on  each  type 
of  clay  must  be  made  with  parallel  observations  on  synthetical  mixtures 
of  pure  minerals.  A  few  such  observations  will  be  made  in  subsequent 
paragraphs. 

Mineralogical  Composition  of  Clays. 

Clay  is  a  heterogeneous  aggregation  of  minerals  in  which  kaolin  is 
present '  in  sufficient  quantities  to  give  to  the  mass  its  characteristic 
physical  properties.  If  kaolin  is  not  present  in  sufficient  quantities  to 


204 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


do  this  the  mass  should  not  be  called  clay.  If  limestone  contains  some 
kaolin  entrapped  mechanically,  the  mass  is  a  limestone,  notwithstanding 
the  fact  that  it  contains  a  considerable  quantity  of  kaolin,  for  it  looks 
and  behaves  in  every  way  like  limestone.  But  if  the  lime  should  be 
dissolved,  as  we  know  has  often  been  the  case,  until  the  material  is  lar¬ 
gely  composed  of  kaolin,  this  residual  mass  is  clay. 

There  is  a  commercial  modification  of  this  definition  that  involves 
its  economic  use  or  value.  For  instance,  if  the  mass  should  contain 
iron  in  sufficient  quantities  to  render  it  a  commercial  source  of  iron, 
the  mass  is  more  properly  called  an  iron  ore,  just  as  a  limestone  impreg¬ 
nated  with  zinc  or  lead  is  termed  a  zinc  or  lead  ore.  Aside  from  cera¬ 
mic  consideration,  a  clay  containing  iron  or  any  other  substance  of  com¬ 
mercial  value  in  sufficient  quantities  to  allow  of  its  being  considered  a 
source  of  that  substance  from  a  commercial  standpoint  would  be  con¬ 
sidered  an  ore. 

COMPLEXITYr  OP  MINERALOGICAL  COMPOSITION  OF  CLAY. 

In  the  section,  the  “geology  of  clays,”  Professor  Eolfe  has  set  forth 
in  detail  the  most  accepted  theory  of  the  origin  of  clays,  the  effective 
agencies  of  rock  decomposition,  and  the  manner  in  wdiich  these  agencies 
operate.  It  ha^,  been  shown  clearly  that  the  residual  mass  resulting 
from  rock  decomposition  may  be  comprised  of  a  variety  of  silicates,  the 
.  kind  and  number  of  silicates  formed  being  dependent  upon  the  con¬ 
ditions  attending  the  rock  decomposition.  Professor  Cook,1  after  giving 
the  analyses  of  several  of  the  New  Jersey  kaolins  that  differ  widely  in 
chemical  composition,  remarks : 

“The  examples  above  stated  prove  conclusively  that  clays  are  not 
altogether  uniform  in  composition,  even  after  throwing  out  all  the  ac¬ 
cidental  or  foreign  constitutents.  Either  the  essential  kaolinite  is  not 
constant ,  or  our  clays  consist  of  this  mineral  mixed  in  varying  propor¬ 
tions  with  other  hydrous  silicates  of  alumina.  Inasmuch  as  the  greater 
number  of  the  rich  fire  and  ware  clays  of  the  State,  and  also  others 
which  have  been  here  examined,  do  correspond  very  closely  to  the  com¬ 
position  and  formula  assigned  to  this  mineral,  the  latter  explanation  is 
more  plausible  " 

After  nearly  thirty  years  of  constant  research  Dr.  Cook’s  problem 
is  no  nearer  solution;  for  Dr.  Clark  in  Bull.  125  of  the  TJ.  S.  Geological 
Survey,  suggests  that  there  are  seven  possible  combinations  of  alumina, 
silica  and  water  of  combination,  which  might  form  crystalized  kaolin. 

Professor  Eolfe  has  shown  that  a  pure  kaolin  can  be  formed  only  by 
the  decomposition  of  rocks  that  consist  almost  altogether  of  feldspathic 
or  other  highly  aluminous  minerals,  together  with  comparatively  unde- 
composed  minerals  like  quartz  and  mica,  that  are  in  large  part  separ¬ 
able  from  the  kaolinite  grains  by  elutriation.  If  the  parent  rock  con¬ 
tains  iron  or  other  metallic  oxide  bearing  minerals  the  residual  kaolin 
will  be  contaminated  with  these  coloring  oxides  in  such  a  manner  as  to 
render  its  purification  by  elutriation  impossible.  If  it  is  impossible  to 
determine  the  mineralogical  constitution  or  makeup  in  the  former  case, 

l  Report  on  the  Clay  Deposits  of  New  Jersey,  Geol.  Surv.  of  N.  J.  1878,  pp. 
269-272. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK. 


205 


where  the  residual  deposit-  is  largely  1  pure  kaolin,  contaminated  only 
with  substances  that  are  separable  in  running  water,  it  is  obvious  that  in 
the  latter  case,  that  of  the  impure  deposit,  the  problem  is  far  more  com¬ 
plicated. 

The  difficulty  of  determining  the  mineralogical  composition  of  a  clay 
.is  increased  many  fold  in  the  case  of  those  that  have  been  transported 
from  the  place  of  origin  and  contaminated  with  a  heterogeneous  as¬ 
sortment  of  inorganic  materials  encountered  en  route.  Shales  may 
vary  so  widely  in  their  mineralogical  constitution  that  in  one  case  the 
mass  may  be  highly  ferruginous,  in  another  highly  calcareous,  and  so 
on,  depending  upon  the  amount  and  kind  of  contaminating  substance. 
Because  the  shale  is  highly  calcareous  it  does  not  follow  that  it  is  a  simple 
mixture  of  calcium  carbonate  and  kaolin,  but  rather  that  the  predomin¬ 
ant  adulterant  is  calcium  carbonate.  Silica,  iron  compounds,  etc.,  may 
be  and  usually  are  present  in  considerable  quantities  in  the  calcareous 
shales.  The  nearest  that  geologists  or  ceramists  have  come  to  deter¬ 
mining  what  inorganic  substances  are  present  in  a  given  shale,  is  sim¬ 
ply  to  say  that  it  is  calcareous  or  silicious,  etc.  It  has  not  been  found 
possible  to  determine  the  mineralogical  composition  of  any  of  the  com¬ 
plex  secondary  clays  either  by  the  microscope  or  by  chemical  analysis 
Approximation  to  the  mineralogical  composition  of  the  purer  secondary 
clays  like  the  ball  clays,  is  made  possible  by  “rational  analysis,”  in 
which  the  differentiation  of  the  minerals  depends  upon  their  relative 
solubility  in  sulphuric  acid,  yet  by  this  method  it  is  incorrectly  assumed 
that  only  three  mineral  components  are  present,  i.  e.,  clay  substance, 
felds|)ar,  and  quartz,  and  the  results  are  forced  to  tally  with  this  as¬ 
sumption. 

It  is  obvious,  therefore,  that  an  attempt  to  distinguish  the  minerals 
that  occur  commonly  in  clays  would  be  useless  in  discussing  the  min¬ 
eralogical  composition  of  clays  in  general,  and  much  more  so  in  the  case 
of  any  particular  clay. 

Granted  that  it  would  be  possible  to  make  a  fairly  accurate  mineral¬ 
ogical  analysis  of  a  clay,  it  is  doubtful  if  our  present  knowledge  would 
enable  us  to  predict  its  working  qualtities  or  even  its  fusibility  with 
accuracy.  When  it  is  considered  that  a  mixture  of  40  per  cent  quartz 
and  60  per  cent  feldspar  has  approximately  the  same  pyrometric  value 
as  feldspar  taken  alone,  and  that  both  have  like  effect  on  the  green  prop¬ 
erties  of  clay,  some  idea  of  the  complexity  of  the  problem  is  apparent. 
What  is  true  of  a  mixture  of  feldspar  and  flint  is  true  of  a  large  number 
of  pairs  of  other  minerals.  What  is  true  of  minerals  when  considered 
in  pairs,  is  true  to  a  larger  degree  when  taken  in  a  multiple  combin¬ 
ation.  It  does  not  require  much  imagination  to  see  where  one  would  be 
led  if  it  should  be  required  to  predict  the  fusion  behavior  or  a  hetero¬ 
geneous  mixture  of  a  large  number  of  minerals. 

This  sort  of  a  study  is  of  value  and  in  fact  is  now  looked  upon  as  a 
necessity  in  the  compounding  of  artificial  mixtures  of  clays  and  min¬ 
erals  for  pottery  purposes,  but  in  these  cases  the  operator  is  dealing 


206 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


with  substances  the  mineral  character  of  which  is  to  a  large  degree 
known,  and  he  is  mixing  these  minerals  in  predetermined  proportions. 
He  has  in  this  case  a  synthetical  mixture  of  known  mineralogical  con¬ 
stitution  adn  of  comparative  simplicity  (containing  at  the  most  not 
more  than  four  or  five  different  minerals,  and,  therefore,  his  practical 
experience  ought  to  enable  him  to  predict  its  physical  and  pyro-chem- 
ical  behavior.  In  the  case  of  nature’s  mixtures,  however,  man  has  at 
present  no  way  of  determining  their  mineralogical  constitution,  and 
must  depend  upon  an  actual  test  for  obtaining  a  knowledge  of  the 
working  properties  of  the  mixture. 

To  illustrate  these  difficulties  reference  might  be  made  to  the  fire 
clays,  which  are  comparatively  pure  clay  substance  or  at  least  rela¬ 
tively  simple  mixtures  of  mineral  ingredients.  It  has  been  shown1  that 
in  plotting  the  position  that  indicates  the  relative  fusibility  of  the  clays 
on  the  basis  of  their  alumina-silica  ratio  in  reference  to  the  position  oc¬ 
cupied  by  a  synthetical  mixture  having  a  similar  alumina-silica  ratio, 
no  concordant  relation  existed  between  them.  Further  the  difference 
between  the  No.  1  and  the  No.  2  fire  clays  of  the  usual  clay  workers’ 
classification  having  practically  the  same  ultimate  fusion  point,  but 
differing  from  one  another  in  the  manner  of  vitrification  is  no  doubt 
explainable  either  on  account  of  difference  in  mineralogical  composi¬ 
tion  or  character  of  grains.  What  is  true  in  the  case  of  simple  mineral 
mixtures  like  the  fire  clays  would  be  greatly  exaggerated  in  the  case  of 
the  exceedingly  complex  mixtures,  such  as  most  of  the  shales  and  sur¬ 
face  clays. 

Ultimate  Chemical  Composition. 

By  ultimate  chemical  composition  is  meant  the  percentage  by  weight 
of  the  several  oxides  of  the  elements  that  occur  in  clay  irrespective  of 
their  original  state  or  combination.  Ordinary  chemical  analyses  are  re¬ 
ported  in  terms  of  so  much  silican  oxide,  aluminium  oxide,  calcium 
oxide,  etc.  All  are  more  or  less  familiar  with  such  analyses,  and  not  a 
few  brick  manufacturers  have  had  repeated  analyses  of  their  clays  made 
by  chemists.  The  reports  they  received  are  what  is  known  as  the 
“Ultimate  Chemical  Analysis,”  in  contradistinction  to  the  rational 
analysis,  that  gives  the  supposed  approximate  percentage  of  clay  sub¬ 
stance,  feldspar  and  quartz  in  the.  clay,  instead  of  the  individual  ox¬ 
ides  of  which  these  substances  are  composed. 

The  persistent  belief  in  the  value  of  an  ultimate  chemical  analysis 
on  the  part  of  layman  and  scientist  alike  is  a  not  wholly  unwarranted 
compliment  to  the  science  of  chemistry.  It  cannot  be  denied  that  there 
is  some  slight  foundation  for  this  unflinching  confidence  in  the  value 
of  an  ultimate  chemical  analysis,  but  it  is  equally  true  that  even  after 
these  many  years  of  constant  research  by  scientists  the  world  over,  very 
little  advance  has  been  made  in  ability  to  interpret  the  facts  that  ought 


1  Preliminary  Report  on  Fire  Clays,  State  Geol.  Surv.,  Bull.  4,  p.  138. 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  207 

to  be  disclosed  by  such  analyses.  Because  so  many  chemists,  as  well 
as  laymen,  do  not  seem  to  understand  the  difficulties  that  attend  the 
interpretation  of  such  an  analysis,  a  brief  review  of  a  few  of  the 
recorded  facts  will  be  given. 

In  1868,  Richters1  in  his  classic  work  entitled  “Refractoriness  of 
Clays,”  promulgated  laws  in  regard  to  the  fluxing  effect  of  the  various 
elements  in  simple  mixtures  at  high  heats  that  are  now  known  as  the 
Richters  laws.  In  1895,  E.  Cramer  published  in  the  “T'hon-Industrie 
Zeitung”  a  review  of  Richter’s  work  confirming  his  laws  in  every  re¬ 
spect.  The  fluxing  behavior  of  the  various  bases  according  to  Richter’s 
laws  are  shown  in  the  following  curves.  Figs.  18  to  21. 


K20  Na^O  CaO  MgO  FeO 


Fig.  18.  Diagram  showing  operations  of  fluxes  in  kaolin,  using  equal  parts  of  each.  (Ex- 
ariple.  kaolin=98^,  K2Q=2$.) 


lFrom  lecture  notes  by  Prof.  Edward  Orton,  Jr. 


208 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


K20  Na2Q  CaO  MgO  FeO 


Fig.  19.  Diagram  showing  operation  of  fluxes  in  kaolin  using  fractions  of  their 
weights;.  (Example;  Ala032Si0,=222.8  at.  wt.  KaO  94.22  mol.  wt.  KaO  mixture 

20  1st  vertical  line. 


atomic 

=222.8+ 


PUR1>Y] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


209 


Fig.  20.  Diagram  showing  operation  of  fluxes  on  Al203+2Si02+J4Si02  mixtures  using 

equal  weights  of  each. 


—14  G 


210 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


F.  21.  Diagram  showing  the  result  of  Richter’s  investigation  of  various  oxides. 

From  Richters5  and  Cramer’s  investigations  it  is  learned  that  the 
order  of  fusibility  of  the  different  oxides  in  simple  clay  mixtures  is  as 
shown  in  the  first  column  of  the  following  table.  In  the  presence  of 
free  silica  the  order  is  changed  somewhat;,  as  is  shown  by  contrasting 
the  order  given  in  the  third  column  with  that  given  in  the  first.  Go¬ 
ing  to  the  other  extreme  of  silicate  mixture,  that  of  glazes,  the  order  of 
the  fluxing  effect  of  the  various  oxides  is,  according  to  Seger1  as  given 
in  the  fifth  column. 

Seger  further  says:  “The  law  established  by  Richter  and  Bischof, 
concerning  the  fusibility'  of  clays,  ‘that  equivalent  proportions  of  fluxes 
exert  an  equal  influence  on  the  fusibility,5  and  which  appears  to  be  ap- 


l  Seger’s  cpllected  works.  Vol.  2,  A.  C.  S.  Translatun,  p.  568. 


PURDY] 


QUALITIES  OF  fcLAYS  FOR  MAKING  PAYING  BRICK. 


211 


Table  XXV. 


Showing  fluxing  behavior  of  the  various  oxides  in — 


Pure  Kaolin. 

Kaolin+^j  mol.  flint. 

Glazes. 

Oxide. 

Molecular 

weight. 

Oxides. 

Molecular 

weight. 

Oxides. 

Molecular 

weight. 

Magnesia . 

40 

Magnesia.. 

49 

Lead . 

222 

Calcium . 

56 

Iron . 

72 

Barium. . . . 

153 

Iron . 

72 

Calcium. .. 

56 

Potash  .... 

94 

Soda  . 

62 

Soda . 

62 

Soda . 

62 

Potash . 

94 

Potash  .... 

94 

Zinc . 

81 

Lime . 

56 

Magesia... 

40 

Alumina... 

102 

proximately  correct  for  the  very  high  temperatures  employed  in  clay 
testing,  and  for  the  very  small'  quantities  of  the  fluxes  coming  into  ac¬ 
tion  in  the  clays,  has  no  bearing  on  the  glasses  and  glazes,  far  richer  in 
fluxes  and  melting  at  far  lower  temperatures.” 

Ludwig1  having  made  similar  studies  with  more  complex  mixtures 
summarizes  his  results  as  follows : 

“ First — Richters’  law  is  a  special  case  of  the  general  law  of  dilute  solu¬ 
tions. 

Second — This  law  is  restricted  by  the  following  correlations: 

a.  It  applies  only  to  very  dilute  solutions,  that  is,  clays  with  a  small 
amount  of  fluxes  and  not  to  brick  clays  or  glazes. 

b.  Is  assumes  intimate  mixtures. 

c.  Iron  shows  a  different  effect,  due  to  its  two  stages  of  oxidation,  since 
one  molecule  of  ferric  oxide  corresponds  to  two  molecules  of  ferrous  oxide. 
A  given  percentage  of  iron  contains  fewer  molecules  of  ferric  oxide  than 
of  ferrous  oxide,  since  the  former  has  a  higher  molecular  weight.  On 
changing  to  the  ferrous  oxide  the  number  of  molecules  is  doubled,  and 
hence  the  fluxing  action  is  doubled. 

Third.  The  analysis  of  a  fire  clay  is  of  great  importance  in  estimating 
its  refractioriness. 

Fourth.  The  estimation  of  refractioriness  by  means  of  the  percentage  of 
alumina  and  fluxes  leads  to  erroneous  results.” 

From  the  above  citation  it  must  be  concluded  that  the  fluxing  power 
of  a  given  oxide  is  affected  very  materially  by  the  kind  and  number  of 
oxides  present,  as  well  as  their  chemical  combination,  degree  of  hydra' 
tion,  oxidation,  etc.  The  facts  gleaned  from  a  study  of  the  fluxing 
effect  of  a  single  oxide  in  a  simple  mixture  do  not  necessarily  hold 
true  in  the  same  degree  in  complex  mixtures.  It  is  well  known  in 
glaze  manufacture,  for  instance,  that  a  mixture  of  several  fluxes  pro¬ 
vokes  greater  fusibility  than  a  mixture  of  any  two  or  them.  What  is 
true  of  glazes  is  likewise  true  of  clays. 

The  difficulty  of  interpreting  the  results  of  chemical  analyses  is  more 
largely  due  to  a  lack  of  experimental  evidence  on  the  fluxing  behavior 
of  known  complex  mixtures.  Interpretation  of  the  facts  concerning 

1  Thon-Industrie  Zeitung,  Vol.  XXVIII,  p.  773,  190*4.  Abstracted  by  Bleininger, 
A.  C.  S.,  p.  275.  Trans.  VII,  p.  275,  1905. 


MELTING  POINTS  EXPRESSED  IN  CONES 


212 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


a  given  mixture  is  impossible  until  there  is  more  known  about  mixtures 
of  the  same  component  substances  in  different  proportional  combina¬ 
tions.  For  example,  Seger1  has  shown  that  the  fusibility  of  mixtures 
of  pure  AM).  and  silica  as  determined  by  Bischof  can  be  represented 
graphically  as  in  Fig.  22. 


Fig.  22.  Seger’s  Si02—Al203  curve. 


1  Seger’s  collected  writings,  Vol.  I,  p.  545,  A.  C.  S.  Trans. 


PURDY]  QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BRICK.  213 

Two  important  facts  are  shown  in  these  curves. 

First.  That  the  kaolin-silica  mixtures  are  more  fusible  than  the  alumin¬ 
ium  and  silicon  oxide  mixture  of  an  equivalent  chemical  composition. 

Second.  That  kaolin  containing  58.2  per  cent  flint  practically  the  same 
fusibility  as  one  containing  88  per  cent,  while  the  kaolin-silica  mixtures 
containing  percentage  of  silica  between  these  two  limits  are  more  fusible 
than  either. 


SEGER  COXES 


Fig.  23.  Melting  points  of  mixtures  of  magnesite  and  Zettlitz  kaolin.  (After  Rieke. ) 

Dr.  Rieke2  has  shown  that  magnesite  will  flux  kaolin,  as  is  shown  in 
Fig.  23.  From  Dr.  Rieke’s  results  it  would  seem  that  a  mixture  of 
85  per  cent  kaolin  and  15  per  cent  magnesium  carbonate  has  approx¬ 
imately  the  same  fusion  point  as  43  per  cent  kaolin  and  57  per  cent 
magnesite. 

Dr.  Mellor1  has  shown  a  similar  fusion  phenomenon  with  mixtures 
of  feldspar  and  quartz,  as  exhibited  in  Fig.  24. 

Surprising  as  are  the  facts  shown  in  these  three  curves,  there  has 
been  but  very  little  effort  to  determine  similar  relations  between  the 
several  pairs  of  oxides  and  compounds,  and  practically  none  to  demon¬ 
strate  the  fusion  behavior  of  the  several  oxides  and  compounds  in 
triple  and  quadruple  combinations,  and  yet  this  is  the  very  data  that 
must  be  worked  out  before  much  can  be  accomplished  in  the  interpre¬ 
tation  of  a  chemical  analysis.  When  ceramic  technology  reaches  this 


1  “Brick,”  p.  170,  Oct.  1906. 

2  Trans.  Eng.  Cer.  Soc.,  p.  51,  1904-5. 


214 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  P 


Fig.  24.  Mellor’s  fusion  curve  for  flint— feldspar  mixtures. 


state  of  progress  an  explanation  can  perhaps  be  made  regarding  the 
fact  that  in  some  cases  the  admixture  of  the  refractory  kaolin  will  cause 
a  lowering  of  the  fusion  point,  while  the  admixture  of  a  flux  such  as 
feldspar  to  the  same  mixture  raises  the  fusion  point.2 

In  the  following  Tables  XXVI  and  XXVII  will  be  found  the  per¬ 
centages  of  the  various  oxides  into  which  the  clays  considered  in  this  re¬ 
port  have  been  resolved.  In  Tables  XXVIII  and  ^vXIX  will  be  found 
the  molecular  composition  of  the  clays  as  calculated  from  the  analysis. 
In  Table  XXX  will  be  found  the  results  of  a  rational  analysis  of  clays 
now  used  for  paving  brick  manufacture  in  the  State. 

Xo  attempt  to  interpret  this  data  can  be  made  at  this  time.  After 
a  discussion  of  the  pyro-chemical  properties  of  clays  this  data  will  be  re¬ 
ferred  to  with  the  endeavor  to  show  all  the  possible  relation  there  may  be 
developed  between  the  physical,  chemical  and  pyro-chemical  properties 
of  clays. 


1  See  A.  C.  S.  Trans.',  Vol.  V,  p.  158. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


215 


Table  XXVI. 


Si02 

Fe203 

A1203 

CaC 

MgO 

Na20 

K20 

Moisture. 

Ignition. 

FeO 

Ti02 

S 

K 

1  .; 

63.36 

1.80 

15.43 

.93 

1.58 

.56 

3.28 

.48 

1 

6.99 

4.02 

1.00 

27 

K 

3  .. 

59.34 

3.26 

15.36 

.76 

1.82 

.80 

3.82 

.29 

7.89 

3.84 

1.31 

.16 

K 

4  .. 

60.31 

5.04 

17.74 

.41 

1.96 

1.07 

2.88 

.81 

6.71 

1.96 

.84 

.14 

K 

5  .. 

63.43 

1.52 

16.89 

1.00 

2.11 

.20 

2.03 

.46 

5.97 

4.24 

1.07 

.11 

K 

6  .. 

63.62 

3.02 

16.28 

.63 

1.44 

1.50 

2.60 

.38 

5.88 

2.90 

.96 

.11 

K 

7  .. 

59.86 

1.42 

17!  43 

1.05 

2.32 

.18 

2.80 

.20 

6.35 

5.10 

1.61 

.13 

K 

14  .. 

64.09 

2.65 

14.16 

1.69 

1.64 

.77 

2.90 

.51 

6.47 

3.16 

.89 

.24 

K 

15  .. 

58.03 

2.91 

17.72 

1.42 

1.43 

1.40 

2.66 

.97 

6.47 

5.77 

1.02 

.25 

F 

1.  .. 

58.52 

4.99 

15.67 

1 

1.05 

1 

1.45 

1.48 

2.94 

2.02 

7.72 

1 

3.37 

.96 

.32 

Table  XXVII. 


Si02 

ai2o3 

Loss  on 
Ignition. 

Fe203 

CaO 

MgO 

Na20K20 

Moisture. 

K  2 . 

63.35 

16.27 

4.75 

7.56 

1.01 

1.33 

3.80 

.31 

K  8 . 

60.89 

16.40 

8.18 

8.20 

.55 

1.61 

4.15 

.27 

K  9 . 

68.50 

16.98 

3.54 

5.77 

.99 

1.71 

2.97 

.50 

K  10 . 

58.35 

18.09 

7.02 

6.14 

1.20 

2.03 

4.58 

.81 

K 11 . 

55.18 

19.22 

10.45 

8.19 

.56 

1.67 

2.85 

1.02 

K  12 . 

54.37 

23.61 

10.09 

6.14 

1.58 

1  61 

2.78 

.60 

K 13 . 

57.09 

19.07 

7.97 

7.92 

.80 

1.91 

4.69 

.43 

S  1 . 

55.02 

20.35 

9.40 

6.26 

.87 

1.70 

3  64 

.83 

S  2 . 

56.29 

20.32 

4.39 

7.90 

.48 

2.01 

4.46 

.•79 

R  1 . 

58.42 

25.05 

8.08 

3.04 

.46 

1.52 

2.30 

1.29 

R  2 . 

63.41 

18.61 

4.86 

5.82 

41 

1.16 

3.60 

.68 

R  3 . 

58.57 

20.40 

5.95 

7.40 

.63 

1.37 

3.27 

1.06 

R  4 . 

55.51 

21.81 

8.00 

7.66 

.56 

1.63 

3.56 

.02 

H  20 . 

47.29 

15.51 

13.11 

4.80 

7.33 

6.19 

3.71 

1.31 

H  23 . 

55.37 

21.40 

8.75 

6.72 

1.76 

.65 

2.41 

3.39 

H-II . 

56.25 

18.79 

7.01 

8.02 

2.39 

1.33 

4.61 

1.49 

H-16 . 

60.93 

17.93 

5.73 

8.12 

1.33 

.91 

5.01 

.55 

H-17 . 

56.56 

12.64 

6.02 

13.56 

2  22 

2.75 

4.82 

3.70 

H-18 . 

39.91 

16.43 

21.20 

4,80 

7.57 

5.08 

3.71 

.86 

H-21 . 

48. 41 

18.31 

12.79 

6.06 

5.73 

3.13 

5.65 

.79 

G-II . 

63.42 

16.24 

5.14 

6.62 

1.64 

1.87 

4.83 

.86 

B-II . 

60.31 

19.11 

6, 70 

6.14 

2.73 

1.73 

1.44 

3.05 

Ill . 

68.15 

12.89 

5.08 

7.52 

1.02 

.59 

2.93 

1.57 

J-II . 

62.70 

16.95 

6.76 

8.98 

1.17 

1.47 

3.03 

.98 

L-II . 

58.62 

17.74 

6.66 

8.48 

1.26 

.98 

‘  3.92 

2.55 

i 

Table  XXVIII. 


Sample 

No. 

Location. 

CaO  MgO 

k2o 

Na20 

FeO 

Fe2Os 

ai2o3 

Si20 

Ti02 

K  1 . 

Alton,  Ill . . 

0.110 

0.261 

0.231 

0.059 

0.369 

0.074 

1.00 

6.98 

0.083 

K  3 . 

Albion,  Ill  . 

0  090 

0.302 

0.270 

0.086 

0.354 

0.135 

1.00 

6.57 

0.108 

K  4 . 

Springfield,  Ill . 

0.012 

0.282 

0  178 

0.099 

0.156 

0.181 

1.00 

5.78 

0.060 

K  5 . 

Rdwardsville,  Ill . 

0.108 

0.319 

0.130 

0.020 

0 . 356 

0.057 

1.00 

6.38 

0.081 

K  6 . 

Galesburg,  111  . 

0.070 

0.225 

0.173 

0.152 

0.252 

0.118 

1.00 

6.64 

0.075 

K  7 . 

Streator  P.  B.  Co . 

0.109 

b.  339 

0.174 

0.017 

0.414 

0.052 

1.00 

5.84 

0.118 

K  14 . 

Western  Brick  Co . 

0.217 

0.295 

0.222 

0.089 

0.309 

0.119 

1.00 

7.69 

0.080 

K  15 . 

Barr,  Streator  Ill . 

0.146 

0.206 

0.163 

0.130 

0.461 

0.103 

1.00 

5.57 

0.073 

F  1 

Danville  P.  B.  Co . 

0.122 

0.236 

0.204 

0.155 

0.305 

0.203 

1,00 

6.35 

0.078 

21 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[bull.  no.  9 


Table  XXIX. 


Sample 

No. 

Location. 

CaO 

r 

I 

MgO  KNaO 

Fe203 

|  A1203 

SiO* 

K  2 . 

St.  Louis,  Mo . 

0.113 

0.208 

0.305 

0.296 

1.00 

6.62 

K  8..  .. 

0.056 

0.250 

0.331 

0.319 

1*00 

6*31 

K  9 . 

Crawfordsville,  Ind . 

0.098 

0  257 

0  229 

0  217 

1  00 

6  86 

K  10  .... 

Terre  Haute,  Ind . 

0.121 

0.286 

0.331 

0.216 

1.00 

5T8 

K  11  .... 

Brazil,  Ind . 

0.053 

0  222 

0.194 

0.272 

1.00 

4.88 

K  13  .... 

Clinton,  Ind . 

0.076 

0 !  255 

0.216 

0.265 

1.00 

5.09 

SI . 

Moberly,  Mo.... 

0.078 

0  213 

0.234 

0.196 

1.00 

4.60 

S2. ... 

Kansas  City,  Mo. 

0.043 

0.252 

0.287 

0.248 

1.00 

4  71 

R  1 . 

Nelsonville,  O . 

0.033 

0.156 

0Y20 

0  077 

1  00 

3*96 

R  2 . 

Portsmouth,  O . 

0.040 

0.159 

0  253 

0  199 

L00 

5.79 

R  3 . 

Canton,  O.  (Imp.) . 

0.056 

0.171 

0  209 

0  231 

1.00 

4 '88 

R  1 . 

Canton,  O.  (Metro) . 

0.047 

0.196 

0  213 

0  224 

1  00 

4  33 

K  12  .... 

|  Brazil  Fire  Clay . 

0.121 

0.174 

0.154 

0.166 

1.00 

3.91 

H-ll . 

Topeka,  Kan . 

0.232 

0.181 

0  321 

0  272 

1  00 

5.09 

H  16 . 

Peoria,  Ill . 

0.135 

0.129 

0  365 

0  289 

1.00 

5.78 

H  17 . 

LaSalle,  Ill . 

0.320 

0  556 

0  499 

0.684 

1.00 

7.96 

H  18..  . 

Sterling,  111 .  . 

0  839 

0.788 

0  295 

0  186 

1  00 

4  13 

G-II  .... 

Coffeyville,  Kan . 

0.184 

0.294 

0.390 

0.260 

1.00 

6.64 

I-II . 

Caney,  Kan . 

0.144 

0.117 

0.297 

0.372 

1.00 

8.99 

J-  II. 

Pittsburg,  Kan  ..  ..  . 

0  122 

0.221 

0  234 

0.378 

1.00 

6.29 

L-II. 

Lawrence,  Kan 

0  129 

0  141 

0  289 

0.305 

1.00 

5.62 

B-1I. 

Atchison,-  Kan. 

0  260 

0.231 

0.099 

0.205 

1.00 

5.49 

H  20.... 

Savanna,  Ill . 

0.861 

1.017 

0.313 

0.197 

1.00 

5  18 

H  21 . 

Galena,  Ill .  . . 

0.570 

0.436 

0.404 

0  211 

1  00 

4  50 

H  23 . 

Carbon  Cliff  Shale . 

0.149 

0.074 

0.147 

0.200 

1.00 

4.39 

Table  XXX. 
Rational  Analysis. 


Sample 

No. 

Clay 

Substance. 

Quartz. 

Feldspar. 

Phos. 

Carbon. 

Soluble 

Salts. 

K  1 . 

35.90 

46.60 

17.50 

.093 

1.44 

.13 

K  3 . 

48.00 

26.74 

25.26 

.078 

1.50 

.14 

K  4 . 

43.32 

43.66 

13.02 

.024 

.72 

.04 

K  5 . 

38.92 

46.54 

14.54 

.090 

1.26 

.08 

K  6 . 

41.02 

39.98 

19.00 

.067 

.63 

.38 

K  7  . 

33.14 

49.36 

17.50 

.079 

.71 

Trace 

K  14 . 

25.28 

48.54 

16  18 

.069 

1.01 

.04 

K  15  . 

53.36 

22.82 

23.82 

.125 

.90 

.27 

F  1 . 

51.12 

29.38 

19.50 

.077 

.92 

.14 

PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


217 


PRO-PHYSICAL  AND  CHEMICAL  PROPERTIES  OF  PAV¬ 
ING  BRICK  CLAYS. 

[By  Ross  C.  Purdy.] 

INTRODUCTION. 

Relations — In  the  discussion  of  the  physical  properties  of  clays  it  was 
shown  that  there  is  a  possibility  of  making  some  correlations  between 
the  several  physical  factors.  It  was  also  demonstrated  that  the  physical 
properties  affect  the  adaptation  of  clays  to  processes  of  manufacture. 
No  relation  was  found  to  exist  between  the  chemical  composition  and 
working  properties,  so  no  attempt  was  made  to  correlate  the  chemical 
and  physical  properties. 

We  are  now  to  consider  those  properties  of  clays  which  are*  manifested 
in  the  process  of  burning,  and  it  is  here  that  we  should  be  able  to  trace 
the  combined  effect  of  the  physical  and  chemical  properties.  In  burn¬ 
ing,  the  physical  and  chemical  properties  of  raw  clays  surely  operate  as 
causes  having  as  effects  the  pyro-physical  and  pyro-chemical  proper¬ 
ties.  If,  knowing  the  physical  and  chemical  composition  of  the  raw 
clays  and  the  pyro-physical  and  chemical  effects  produced  in  burning, 
we  are  not  able  to  trace  a  logical  and  invariable  sequence  between  the 
causes  and  effects,  we  will  be  forced  to  admit:  either  (a)  That  accord¬ 
ing  to  the  data  at  hand,  clays  having  similar  physical  and  chemical 
properties  in  the  raw  state,  may  behave  differently  in  burning,  or,  (b) 
That  it  is  at  present  impossible  to  determine  exactly  the  physical  and 
chemical  condition  of  raw  clays;  or,  (c)  It  is  impossible  to  trace  the 
effect  of  individual  physical  and  chemical  properties  where  so  large  a 
number  of  changes  occur  simultaneously;  or,  (d)  That,  reasoning  from 
analytically  determined  causes  to  observed  effects  is  an  absurdity  if  the 
evidence  does  not  permit  of  a  reverse  reasoning,  i.  e.,  from  effect  to 
cause. 

The  first  case,  that  of  clays  of  similar  character  behaving  differently 
in  burning,  is  forcibly  illustrated  in  the  case  of  fire  clays.  Fire  clays, 
having  similar  ultimate  chemical  composition  and  size  of  grain,  may 
have  radically  different  pyro-physical  behavior.  The  one  may  burn  to 
an  open  porous  mass  at  cone  11,  being  fit  for  fire  brick  purposes;  the 


218 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO  9 


other  may  burn  quite  dense  at  cone  8,  being  fit  for  stoneware,  sewer  pipe, 
etc.  This  fact  was  noted  in  the  preliminary  report  on  fire  clays,1  and 
will  be  illustrated  in  this  report  under  the  topic  “Changes  That  Take 
Place  During  Fusion.” 

The  second  case,  the  impossibility  of  determining  exactly  the  physical 
and  chemical  condition  of  raw  clays,  is  illustrated  by  the  fact  that  in 
the  more' exact  of  the  two  analyses,  the  chemical,  chemists  do  not  claim 
to  be  able  with  ordinary  care  and  attention  to  details,  to  determine  all 
of  the  elements  that  may  be  in  a  clay,  nor  do  they  claim  to  be  able  to 
determine  the  combinations  in  which  these  elements  exist. 

The  third  case,  or  the  impossibility  of  tracing  the  effect  of  several 
changes  in  physical  and  chemical  conditions  which  take  place  simultan¬ 
eously,  is  a  well  recognized  fact.  On  a  rectangular  coordinate  diagram, 
two  changes;  on  a  triangular  coordinate  diagram,  three  changes  in 
properties  can  be  traced  with  accuracy.  No  simple2  method  has  yet 
been  devised  by  which  the  effect  of  changes  in  four  factors  can  be 
traced,  and  it  is  certainly  beyond  the  capacity  of  the  human  mind  to 
follow  the  effects  of  four  or  more  changes,  if  they  cannot  he  plotted 
diagrammatically.  In  the  case  of  several  clays,  no  two  of  which  agree 
exactly  in  their  several  properties,  and  in  all  of  which  there  are  a  great 
many  properties  peculiar  to  the  individual  clays,  it  is  manifestly  beyond 
our  ability*  to  satisfactorily  folloy  even  all  the  known  details.  Broad 
generalizations  with  numerous  and  well-known  exceptions  are  the  best 
that  experimenters  have  been  able  to  make  from  synthetical  mixtures 
of  fairly  pure  clays.  It  is  obvious,  therefore,  that  with  a  heterogeneous 
assortment  of  impure  clays,  conclusions  concerning  the  relation  between 
the  causes  (the  physical  and  chemical  properties  of  raw  clay)  and  ef¬ 
fects  (the  pyro-physical  and  chemical  properties)  cannot  be  other  than 
broad  generalizations. 

The  last  case,  that  of  the  absurdity  of  claiming  validity  for  deduc¬ 
tions  drawn  by  reasoning  from  cause  to  effect  in  cases  where  data  do  not 
permit  of  a  reverse  reasoning,  i.  e.,  from  effect  to  cause,  is  very  nicely 
illustrated  in  the  work  of  Hoffman  and -Desmond3  where  an  attempt 
was  made  to  devise  an  indirect  method  of  determining  the  refractori¬ 
ness- of  clays.  With  a  given  furnace  operating  on  a  predetermined  time- 
temperature  schedule,  they  thought  they  were  successful  in  determining 
the  relative  refractoriness  of  clays  by  toning  up  low  grade  clavs  with 
the  addition  of  refractory  material,  and  toning  down  high  grade  clays 
by  the  addition  of  known  amounts  fluxes,  until  the  clays  had  the  same 
refractoriness  under  the  same  heat  treatment.  This  scheme  worked 

1  State  Gaol.  Surv.  of  Ill..  Year  Book  1906.  p.  138. 

2  Solid  figures  are  used  by  physical  chemists  in  depicting  the  combined  effect 
of  more  than  three  factors,  but  the  drawing  of  such  figures  to  scale  according  to 
given  data  presents  difficulties  which  the  writer,  at  least,  has  been  unable  to  sur¬ 
mount.  Judging  from  the  fact  that  ceramists  and  other  technical  scientists  have 
not  as  yet  used  solid  figures  it  must  be  inferred  that  others  have  also  found  the 
involved  difficulties  insurmountable. 

3  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXIV,  p.  32. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


219 


nicely  until  they  assumed  definite  temperatures  and  attempted  to  pre¬ 
pare  mixtures  that  would  fuse  at  these  temperatures.  In  the  first  in¬ 
stance  they  adopted  a  certain  combination  of  “causes”  and  measured 
the  “effects.”  Tn  the  second  instance  they  adopted  an  “effect”  and  at¬ 
tempted  to  determine  the  combined  “causes”  that  produced  this  effect. 
In  this  they  failed  so  utterly  that  they  abandoned  this  indirect  method 
of  estimating  refractoriness.  If  their  careful  researches  demonstrated 
no  other  fact  than  the  futility  of  attempting  to  draw  conclusions  con¬ 
cerning  the  relation  between  cause  and  effects,  when  the  data  show  this 
relation  operating  only  in  one  direction,  i.  e.,  only  from  cause  to  effect 
or  from  effect  to  cause,  their  work  was  worth  while  and  their  report  a 
valuable  addition  to  ceramic  knowledge. 

Relative  Importance  of  Raw  and  “  Bw'ning”  Properties — It  is  plain 
that  the  physical  properties  of  a  raw  clay  influence  its  behavior  mainly 
in  the  machines  and  dryers.  True,  the  physical  properties  have  their 
influence  on  the  burning  behavior  of  clays,  and,  as  in  case  of  size  of 
grain,  if  the  causes  of  the  physical  properties  were  determinable,  their 
findings  would  be  of  value  in  predicting  and  explaining  the  properties 
developed  in  burning.  Size  of  grain,  as  will  be  shown,  is  an  important 
factor  in  the  case  of  pure  minerals,  but  when  the  grains  do  not  have  a 
homogeneous  mineral  composition,  but  are,  in  the  main,  clots  of  minute 
particles  of  several  minerals,  or  particles  of  the  same  mineral  substance 
cemented  together,  any  data  concerning  the  influence  of  fineness  of 
grain  on  the  properties  developed  in  burning  are  apt  to  be  very  mis¬ 
leading.  Grout’s  analysis  of  the  grains  of  clays,  given - on  pages  - 

shows  that  the  grains  are  not  individual  particles  but  are  aggregates, 

and  Fox’s  results,  cited  on  pages - - —  confirm  the  conclusions  drawn 

from  Grout’s  data.  The  writer  has  ground  impure  clays  until  they 
passed  sieves  of  different  meshes  ranging  from  10  to  200,  molded  the 
clays  -into  cones  and  noted  the  effect  of  fine  grinding  on  the  refractori¬ 
ness  of  the  resulting  masses.  The  difference  between  the  ultimate 
fusion  failure  to  stand  erect  under  high  heat  treatment  of  the  cones 
prepared  from  the  same  clay  but  differing  in  size  of  grain,  was  hardly 
observable.  True  there  was  a  difference  in  that  the  finely-ground  sam¬ 
ples  vitrified  earlier  and  did  not  lag  as  much  in  bending  over  so  that 
they  could  be  said  to  be  a  trifle  less  refractory.  In  no  case,  however, 
was  the  difference  in  refractoriness  between  the  10  and  200  mesh  sample 
of  the  same  clay  more  than  20  to  40  degrees  centigrade,  as  measured  by 
LeChatelier  electric  resistance  pyrometer. 

Indirectly,  fineness  of  grain  affects  the  burned  product  in  that  in¬ 
ternal  fractures  produced  in  drying  and  lamination  in  the  machine 
dies  caused  by  extreme  fineness  of  grain  weaken  the  finished  product. 
These  and  similar  considerations  are  not  properly  considered  under  the 
topic  of  Pyro-physical  and  Chemical  Products. 

The  main  consideration,  in  an  analysis  of  the  influence  of  the  sev¬ 
eral  properties  of  clays,  is  their  influence  on  the  character  of  the  pro¬ 
duct  manufactured  from  the  clays  in  question.  In  the  case  of  paving 


220 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


brick  the  desired  character  of  product  is  toughness  or  resistance  to  im¬ 
pact  and  abrasion.  If  coarse  as  well  as  fine  grained  clays,  plastic  as 
well  as  non-plastic  clays,  and  tough  clays  or  clays  that  show  but  little 
tensile  strength,  can  be  burned  so  as  to  make  tough  bricks,  it  is  obvious 
that  it  will  be  impossible  from  such  physical  data  to  predict  the  char¬ 
acter  of  ware  which  a  given  clay  will  make.  Inability  to  trace  the  in¬ 
fluence  of  so  many  factors  may  be  largely  responsible  for  this  seeming 
lack  of  relation  between  the  physical  properties  of  the  raw  clay  and 
the  properties  of  the  burned  ware,  but  the  fact  remains  that  such  is 
the  case. 

On  the  other  hand  it  can  be  shown  that  there  is  a  possible  or  seem¬ 
ing  relation  between  pyro-physical  and  chemical  properties  and  the 
properties  of  the  burned  ware.  Such  a  relation  has  been  shown  to 
exist  in  the  case  of  fire  clays.  In  the  case  of  paving  brick  clays  there 
is  not  quite  so  distinct  a  relation  between  these  factors,  but  still  it  is 
observable.  The  study  of  the  pyro-physical  and  chemical  changes  pro¬ 
duced  in  clays  by  heat  is,  therefore,  of  considerable  more  importance  in 
the  'study  of  paving  brick  clays  than  the  study  of  the  physical  prop¬ 
erties  of  the  raw  clay. 

DEHYDRATION. 

Nature  of  process — Pure  kaolin,  the  basic  c^y  substance,  contains  in 
round  numbers  14  per  cent  of  water,  chemically  combined.  At  ordin¬ 
ary  drying  heats  the  amount  of  this  chemically  combined  water  in  the 
kaolin  is  supposed  to  be  unaltered.  In  fact,  there  is  experimental  evi¬ 
dence  to  support  the  belief  that  there  is  some  water  mechanically  re¬ 
tained  by  the  clay  even  at  the  highest  heat  ordinarily  attained  in  any 
dryer,  but  this  has  no  relation  to  the  chemically  combined  water. 

Since,  however,  in  the  ordinary  clay  or  shale  but  a  fractional  part 
of  the  whole  is  kaolin,  ranging  from  a  possible  100  per  cent  in  the 
purest  varieties  down  to  25  per  cent  or  less  in  the  more  impure  clays, 
it  is  not  surprising  that  the  amount  of  chemically  combined  water 
varies  greatly  in  the  different  clays.  Even  in  the  purest  it  varies 
to  some  extent,  amounting  in  some  cases  to  more  than  14  per  cent.  In 
these  not  rare  cases  some  other  hydrous  minerals  are  supposed  to  be 
present  that  carry  a  higher  percentage  of  combined  water.  It  is  aside 
from  our  purpose  to  dwell  upon  the  kind  and  nature  of  the  hydrous 
minerals  that  may  occur  in  clay  except  to  note  that,  if  they  occur  in 
the  purest  types  of  clays,  and  especially  those  which  have  not  been  moved 
from  their  place  of  formation,  it  is  reasonable  to  suppose  that  in  a 
heterogeneous  mixture  of  minerals  such  as  shales  seem  to  be,  these 
highly  hydrous  minerals  may  in  some  cases  be  present  in  considerable 
quantities.  Since  each  hydrous  mineral  substance  retains  its  chemically 
combined  water  with  a  tenacity  peculiar  to  itself,  it  follows  that  the 
period  of  dehydration  of  clays  will  vary  with  each  variation  in  quan¬ 
tity  and  kind  of  hydrous  minerals  present.  Likewise  the  physical 
alteration  in  the  mass  at  this  period  will  vary  with  each  variation  in 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAVING  BRICK.  221 

kind  and  quantity  of  hydrous  minerals  present.  Since,  however,  it  is 
impossible  to  gather  reliable  data  as  to  the  mineralogical  constitution 
of  the  impure  clays,  the  quantities  of  these  hydrous  minerals  present 
must  be  mere  speculation.  The  varying  effects  produced  during  the 
period  of  dehydration,  which  probably  originate  in  variable  mineralog¬ 
ical  composition,  are  the  only  known  or  determined  facts  in  the  case. 

From  the  foregoing  considerations  it  is  not  surprising  that  the  tem¬ 
perature  of  dehydration  has  been  considered  as  ranging  from  550/  to 
650°  centigrade  (990  to  1170°  Fahrenheit),  and  that  there  are  Clays 
which  can  withstand  a  heat  treatment  of  16  hours  duration~~alra  tem¬ 
perature  which  will  average  during  this  period  at  least  650°  C.  without 
entire  loss  of  plasticity. 

Six  clays  (K  5,  H  16,  K  8,  K  13,  K  14,  K  15)  after  subjection  to 
a  heat  treatment  supposedly  sufficient  to  affect  complete  dehydration, 
slaked  down  in  water  to  a  red  plastic  mass  similar  to  that  produced  from 
hard  shale  on  weathering.  If  it  is  true  that  on  dehydration  clay  loses 
the  properties  that  cause  the  mass  to  exhibit  plasticity  then  these  clays 
were  not  dehydrated.  If  clays  that  have  been  subjected  to  just  sufficient 
heat  treatment  to  cause  their  complete  dehydration  still  retain  consid¬ 
erable  plasticity,  then  many  will  have  to  change  their  conception  as  to 
the  cause  of  plasticity,  for  surely  nearly,  if  not  all,  of  the  physical 
properties  of  the.  kaolin  particles  must  be  altered  by  dehydration.  These 
six  clays  tested  continued  to  lose  weight  after  this  period.  This  loss 
may  possibly  be  accounted  for  by  the  loss  of  volatile  matter  other  than 
chemically  combined  water.  In  the  absence  of  analytical  data,  however, 
it  was  fair  to  assume  that  this  additional  loss  was  in  part  at  least  due  to 
the  further  expulsion  of  the  chemically  combined  water.  If  this,  as¬ 
sumption  is  correct,  these  cases  would  indicate  that  the  usually  allotted 
range  in  temperature  for  this  period  is  altogether  too  limited.  If  a 
clay  can  withstand  heat  treatment  for  16  hours  at  a  temperature  that 
ranges  from  500°  to  740°  C.,  with  an  average  equal  to  650,  without 
complete  loss  of  its  combined  water,  it  is  fair  to  conclude  that  the  max¬ 
imum  temperature  limit  for  the  dehydration  period  is  above  700°  C. 

Loss  due  to  Constituents  other  than  Combined  Water — The  actual  loss 
in  weight  of  a  clay,  aside  from  loss  of  the  chemical  water,  up  to  this 
temperature  may  in  part,  according  to  Prof.  Orton1  be  accounted  for 
as  follows: 

Vegetable  tissues,  such  as  roots,  leaves,  etc.,  ignite  and  burn  at  about 
300°C. 

Bituminous  matter,  common  to  shales,  ignites  and  burns  between  300 
and  400°C. 

Graphitic  carbon,  does  not  ignite  much  before  500°€. 

Sulphur  distils  from  iron  pyrites  between  400  and  600°C. 

Calcium  carbonate  decarbonizes  between  600  and  1000°C. 

Ferrous  carbonate  decarbonizes  between  350  and  430°C. 


lAmer.  C.  Soc.,  Vol.  V,  p. 


222 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[bull.  no.  9 


The  loss  of  any  or  all  these  constituents  would  not  materially  affect 
the  plasticity  of  clay,  and  in  the  main  these  reactions  would  be  com¬ 
pleted  before  or  at  the  same  time  as  dehydration.  In  caes  of  K  14  be¬ 
fore  referred  to,  they  had  all  evidently  been  completed  before  the  com¬ 
pletion  of  dehydration,  except  perhaps  the  decarbonation  of  the  small 
amount  of  contained  calcium  carbonate.  The  bricks  were  thoroughly 
oxidized  and  normal  salmon-colored  throughout.  In  this  case  the  only 
possible  conclusion  seems  to  be  that  dehydration  of  clay  requires  more 
heat  than  heretofore  supposed. 

It  has  been  demonstrated  by  Hopewood1  that,  aside  from  the  loss  of 
combined  water,  solid  carbon,  carbonic  acid  gas,  sulphur,  etc.,  quite 
a  large  variety  of  acids  and  bases2  are  expelled  from  the  clay  by  vola¬ 
tilization  at  temperatures  below  the  maximum  required  for  complete 
dehydration.  The  evidence  given  to  Hopewood’s  experiments,  together 
with  the  vast  accumulation  of  data  by  agricultural  chemists,  makes  if 
appear  as  though  the  absorbed  as  well  as  the  absorbed  salts  are  seriously 
affected  during  this  period.  Direct  evidence  is  not  at  hand  that  would 
throw  light  on  this  question,  but  the  value  of  such  evidence  is  con¬ 
sidered  by  the  writer  to  be  of  such  importance  that  an  extensive  re¬ 
search  dealing  with  this  subject  has  been  outlined.  It  is  anticipated 
that  the  manner  in  which  this  period  of  burning  (dehydration)  is 
conducted  will  be  found  to  play  a  very  significant  role  in  the  character 
of  the  ware  developed  in  “the  finishing  heats.” 


OXIDATION. 

General  Conditions. 

Definition  of  terms — “Oxidation”  and  “Reduction”  are  chemical 
terms  referring  respectively  to  taking  on  and  giving  off  of  oxygen. 
When  a  piece  of  iron  is  rusting  it  is  becoming  oxidized,  i.  e.,  the  metal 
(Fe)  is  being  converted  to  an  oxide  of  iron  (Fe20s)  which  is  red  in 
color.  Iron  rust  can  be  reconverted  to  the  unoxidized  metallic  state 
again  by  application  of  heat  under  reducing  conditions,  i.  e.,  condi¬ 
tions  that  favor  separation  of  the  metallic  iron  and  oxygen.  When  the 
quantity  of  oxygen  in  combination  is  reduced,  then  it  is  said  that  re¬ 
duction  has  taken  place.  When  the  quantity  of  oxygen  in  combination 
has  been  increased  then.it  is  said  that  oxidation  has  taken  place. 

Evidence  of  reduced  condition  in  raw  clay— “Blue”  clay  and  dark 
gray  shale  owe  their  characteristic  blue  color,  in  the  main,  to  two 
classes  of  substances,  (1)  the  ferrous  compounds,  principally  ferrous  car¬ 
bonate  and  (2)  carbon.  Partially  metamorphosed  carbon  adds  to -a  clay 
mass  its  characteristic  black,  just  as  does  lamp  black  when  added  to 

what  would  otherwise  be  a  white  mass.  Lamp  black,  an  amorphous 

form  of  carbon,  is  the  product  of  decomposition  of  carbon  compounds 

1  Trans.  Eng.  Cer.  Soc.,  1904-5,  p.  37. 

2  R.  K.  Meade  has  shown  analytical  data  in  support  of  his  claims  that  the 
alkalies  in  cement  mixtures  are  expelled  during  the  burning.  “Portland  Cement” 
p.  124. 

J.  W.  Mellor  also  shows  with  data  that  the  loss  of  alkalies  from  fire  clays  fired 
at  1400°C.,  amounts  to  20  per  cent  of  the  total  alkalies  present  in  the  unburned 
clay.  Trans.  Eng.  Cer.  Soc.  Vol.  VI,  p.  130. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAYING  BEICK. 


223 


under  the  influence  of  heat,  resulting  from  conditions  that  prevent  its 
complete  oxidation.  The  carbon  in  shales,  at  one  time  a  part  of  the 
fibrous  tissue  of  living  plants,  was  buried  in  deposits  of  sea  mud,  and  is 
found  today  in  this  same  mud  hardened  into  shale.  Therefore,  the  dark 
iron  compounds  and  the  metamorphosed  remains  of  carbon  compounds 
combine  to  give  the  characteristic  blue  color  to  shales  and  many  fire 
clays. 

Evidence  of  oxidation  in  raw  clay — Where  the  shale  is  covered  with 
only  a  very  thin  “stripping,”  the  color  of  the  upper  three  or  four  feet 
of  the  bank  will  be  red.  In  the  lower  portion  of  these  red  strata  the 
color  shades  off  gradually  into  the  blue  of  the  more  solid  strata  below. 
In  this  red  portion  near  the  top  of  the  bank  the  ferrous  compounds 
have  been  oxidized  to  ferric  compounds  by  the  action  of  the  oxygen 
from  the  atmosphere.  Below  the  belt  of  weathering,  the  clay  retains 
its  blue  color  owing  to  the  fact  that  either  air  cannot  penetrate  to  those 
depths  or  that  its  oxygen  is  largely  spent  before  it  can  reach  the  lower 
limit  of  the  belt  of  weathering.  It  is  observed  that  oxidation  starts 
at  the  surface  and  proceeds  downward.  The  depth  to  which  evidence 
of  oxidation  can  be  seen  depends  upon  the  nature  and  amount  of  the 
oxidizable  mineral  present,  the  solidity  of  the  rock  mass,  the  prevailing 
atmospheric  conditions  and  the  length  of  time  of  exposure. 

Oxidation  of  Clay  in  Buening. 

The  very  same  processes  that  are  effective  in  oxidizing  the  blue  shale 
to  “red  outcrop”  are  operative  in  burning  when  the  blue  clay  brick  is 
converted  into  “salmon  brick.”  In  nature,  at  ordinary  temperatures 
and  under  varying  conditions,  this  oxidizing  process  is  very  slow,  but 
in  the  kiln  at  temperatures  ranging  from  500  to  800°  centigrade,  with 
the  high  draft  that  is  usually  maintained  at  this  early  stage  of  the 
burning,  conditions  under  which  oxidizing  processes  operate  are  very 
much  intensified  and  consequently  comparatively  rapid  in  their  action. 
In  the  case  of  surface  clay,  and  red  clays  generally,  oxidation  is  so 
rapid  that  the  lag  in  time  incident  to  raising  heat  in  a  large  kiln  of 
relatively  cold  ware  is  sufficient  to  complete  the  oxidizing  processes. 

In  the  case  of  many  of  the  shales,  the  time  required  to  completely 
oxidize  the  clay  is  so  much  longer  that  either  the  burner  must  “hold 
the  kiln  at  red  heat”  for  a  time,  or,  especially  in  the  case  of  bricks 
which  have  been  set  wet,  evidence  of  incomplete  oxidation  will  be  very 
evident  when  the  bricks  are  drawn.  The  change  in  color  from  blue  in 
the  “green”  ware,  to  red  in  the  salmon  is  the  result  of  oxidation.  Red 
surface  and  black  centers  are  results  of  incomplete  oxidation.  These 
changes  in  color  are  the  same  indicators  of  oxidation  and  lack  of  oxida¬ 
tion  noted  in  the  case  of  shale  in  the  bank. 

SUBSTANCES  IN  CLAY"  THAT  AKE  AFFECTED  BY  OXIDATION. 

In  general  terms,  the  oxidizable  substances  in  clays  are  carbon  com¬ 
pounds,  carbonates,  nitrates,  sulphites,  etc.  The  most  noteworthy  ox¬ 
idizable  substances  in  clays  are  :  Carbon  and  the  carbon  compounds, 
ferrous  carbonate  and  ferrous  sulphide. 


224 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


Carbon  and  the  Carbon  Compounds — Carbon  is  present  in  practically 
all  of  the  secondary  clays  in  forms  ranging  from  unaltered  vegetable 
matter,  humus  and  its  compounds,  to  the  Jiighly  metamorphic  carbon- 
graphite  in  graphitic  shales.  The  least  altered  carbon  ignites  and  ox¬ 
idizes  most  easily  and  the  highly  metamorphosed  carbon  most  difficulty. 
To  the  decomposing  carbon  compounds  and  their  by-products,  the  or¬ 
ganic  acids,  are  due  many  of  the  physical  properties  of  clays.  It  has 
been  shown  in  earlier  pages  that  organic  acids  are  the  main  agents  that 
cause  deflocculation,  a  condition  that  must  exist  before  plasticity  can  be 
developed.  It  could  be  readily  shown  that  humic  acid  (C20H2O9)  with 
its  peculiar  properties  of  absorbing  and  holding  heat,  moisture  and 
soluble  salts,  is  a  very  active  agent  in  promoting  chemical  changes  in 
the  mineral  ingredients  of  clay,  thus  altering  the  physical  condition 
of  the  mass.  Unaltered  carbon  compounds  and  their  by-products  are, 
therefore,  not  only  easily  oxidized  in  burning,  but  have  been  highly  ben¬ 
eficial  in  that  they  have  promoted  the  development  of  those  physical 
properties  which,  if  the  carbon  is  not  in  excess,  permit  of  easy  manufac¬ 
ture  into  wares. 

The  more  metamorphosed  the  carbon  compounds  the  less  active  they 
are  in  promoting  physical  and  chemical  alterations  in  the  clay  mass  and 
the  more  difficult  are  they  to  oxidize  in  the  kiln.  For  these  reasons  fire 
clays  and  clay  shales  in  which  the  carbons  compounds  have  been  com¬ 
pletely  converted  to  graphite  are— within  small  areas — more  constant 
in  their  properties,  thus  being  more  constant  in  their  working  and 
burning  behavior,  and  at  the  same  time,  more  difficult  to  burn. 

Ferrous  Carbonate — Ferrous  carbonate  occurs  in  clay  in  various  phy¬ 
sical  conditons  and  sizes  of  grain.  Large  concretions — “nigger  heads’* — 
which  are  often  composed  mainly  of  ferrous  carbonates,  are  to  be  seen 
in  most  shale  banks.  Ranging  in  size  from  12  to  18  inches  in  diameter, 
down  to  minute,  almost  microscopic  particles,  these  concretionary  and 
globular  forms  of  ferrous  carbonate  play  a  role  in  burning  clay  wares 
which,  while  most  peculiar,  is  but  little  understood.  The  ferrous  car¬ 
bonates  that  exist  as  finely  precipitated  powder  surrounding  the  other 
mineral  grains  must,  in  burning,  pass  through  the  same  chemical  alter¬ 
ations  as  the  ferrous  carbonate  in  lump  form,  but  under  such- different 
conditions  that  distinction  must  be  made  between  its  behavior  when  in 
these  two  conditions  of  aggregation. 

One  of  these  large  ferrous  carbonate  concretions  pulverized,  pressed 
into  brick  form  and  burned  under  the  same  heat'  treatment  required 
to  produce  pavers  from  the  shale  in  which  the  concretion  was  found, 
produced  a  brigh  red  brick  which  possessed  a  toughness  that  was  equal 
to  that  of  the  brick  made  from  shale.  This  experiment  proved  that  the 
clay  mass  which  is  bound  together  by  ferrous  carbonate  in  a  mass  so 
hard  as  to  wreck  ordinary  crushing  machines  like  dry  pans,  and  contain¬ 
ing  a  comparatively  large  quantity  of  ferrous  carbonate,  can  be  burned 
as  safely  and  into  just  as  good  brick  as  the  softer  shale  containing  but 
a  small  quantity  of  ferrous  carbonate  (3  per  cent  of  total  ferrous  iron.) 


PURDY]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  225' 

Ill  this  brick  made  from  the  crushed  concretion  there  was  practically 
no  carbon,  while  the-  shale  contained  three  quarters  of  one  per  cent. 
While  it  is  true  that  the  carbon  content  of  the  shale  is  so  small  that  no 
difficulty  is  experienced  in  thoroughly  oxidizing  the  mass  under  the.  time 
temperature  schedule  required  to  raise  heat  uniformly  in  a  large  kiln, 
yet  it  is  a  significant  fact  that  occasionally  unoxidized  brick  are  drawn 
from  the  kilns,,  and  that  the  mass  containing  a  large  amount  of  ferrous 
carbonate  was  perfectly  oxidized  under  similar  kiln  treatment. 

Singer1  has  shown  that  the  acid  radical  (CO2)  is  expelled  from  fer- 
ous  carbonate  at  temperature  below  430  C.  The  basic  radical  (FeO) 
would  thus  be  given  ample  time  to  become  thoroughly  oxidized  to  FesO« 
or  FeaO  before  the  temperature  could  be  raised  sufficiently  to  cause  fusion 
between  the  ferrous  iron  and  the  silicates.  Under  normal  kiln  treatment 
complete  oxidation  of  the  iron  would  be  effected,  provided  the  clay  mass 
contained  but  a  small  amount  of  carbon.  In  the  almost  total  absence 
of  carbon,  our  experiment  with  the  concretionary  mass  proved  that  the 
iron  could  be  quite  readily  oxidized.  As  the  carbon  content  increased, 
the  difficulty  in  oxidizing  a  given  amount  of  ferrous  iron  would  increase, 
for  between  carbon  and  oxygen  there  is  a  stronger  affinity  than  between 
iron  and  oxygen.  In  case  there  is  a  high  content  of  both  carbon  and 
ferrous  carbonate,  time  would  have  to  be  allowed  in  burning  to  com¬ 
pletely  burn  out  the  carbon  before  the  heat  is  raised.  If  this  should 
not  be  done  the  ferrous  oxide  would  flux  with  the  silicates  causing  an 
early  fushion  in  the  unoxidized  portion  of  the  brick. 

In  case  the  carbon  is  easily  ignited  and  burns  freely  it  has  been  found 
that  the  fires  in  the  furnaces  have  to  be  drawn,  all  air  supply  shut  off 
and  the  carbon  allowed  to  smolder  until  completely  burned  out.  If 
these  precautions  are  not  taken  in  such  cases,  the  heat  from  the  burning 
carbon  will  raise  the  temperature  in  the  kiln  to  the  point  where  the  fer¬ 
rous  iron  will  be  slagged  with  the  silicates.  In  fact,  the  iron  that  was 
originally  in  an  oxidized  condition  would  be  reduced,  and  the  whole  iron 
content  thus  be  brought  to  its  most  active  fluxing  condition. 

Where  the  carbon  is  less  inflammable,  a  longer  time  would  have  to  be 
allowed  for  its  complete  combustion,  but  such  stringent  precautions 
would  not  have  to  be  taken  as  in  the  case  where  the  clay  contained  more 
inflammable  carbon. 

The  chemical  explanation  of  these  cases  is  that  although  the  CO2 
radical  is  expelled  from  ferrous  carbonate  at  an  early  stage  in  burning, 
the  basic  radical  (FeO)  cannot  receive  the  oxygen  required  to  con¬ 
vert  it  to  its  less  active  fluxing  form,  i.  e.,  to  Fe20s  as  long  as  there  is 
carbon  left  in  the  clay  mass.  Carbon  having  a  greater  affinity  for  oxy¬ 
gen  than  the  ferrous  iron  will  withhold  it  from  the  iron.  If  a  clay  con¬ 
tains  insufficient  carbon  of  an  easily  inflammable  variety,  or,  if  the  car¬ 
bon,  even  though  present  in  quantity,  is  difficultly  inflammable,  time 
must  be  allowed  to  permit  the  oxygen  to  penetrate  the  brick,  for  oxida¬ 
tion  proceeds  from  the  exterior  towards  the  interior  in  a  manner  similar 
to  the  oxidation  of  shale  in  the  bank  from  the  outcrop  downward. 

l  Class  exercise  under  Orton,  Ohio  State  Univ. 


—15  G 


226 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[bull.  no.  9 


High  content  of  ferrous  carbonate  does  not  in  itself  mean  that  trouble 
will  be  experienced  in  oxidation,  nor  does  a  high  content  of  thoroughly 
oxidized  iron  considered  alone  indicate  immunity  from  oxidation 
troubles.  The  substance  that  controls  the  manner  in  which  the  oxida¬ 
tion  .period  of  burning  clay  wares  must  be  conducted  is  carbon.  Burn¬ 
ing  carbon  not  only  will  prevent  oxidation  of  the  ferrous  iron,  but  will 
reduce  the  iron  that  may  have  originally  been  in  a  thoroughly  oxidized 
condition.  It  depends,  therefore,  upon  the  amount  and  form  of  carbon 
present  in  a  given  case,  as  to  whether  in  burning  there  must  be  allowed 
a  short  or  long  oxidizing  period. 

Ferrous  Sulphide — This  very  frequently  occurs  in  clays  as  bright 
3’ellow  or  white  crystals.  The  first  of  these  forms  is  often  mistaken 
for  gold  because  of  its  similarity  in  color.  It  is  commonly  known  as 
“fooTs  gold.”  Mineralogically  it  is  known  as  iron  pyrites  or  marcasite, 
depending  upon  its  crystalline  form. 

If  clay  containing  pyrites  is  loosened  and  allowed  to  weather,  the 
pyrites  will  be  desulphurized.  The  iron  will,  in  the  dry,  oxidize  to  the 
hematite  (FesCb),  or,  if  moisture  i§  present,  to  limonite  (2  FesOs- 
3H2O).  The  sulphur  will  at  the  same  time  oxidize  to  sulphurous  or 
sulphuric  acid.  By  weathering,  therefore,  iron  pyrites  can  be  thoroughly 
oxidized  and  the  sulphurous  and  sulphuric  acid  removed  in  solution  by 
percolating  waters.  These  reactions  require  time,  especially  under  dry 
conditions.  Brick  manufacturers  cannot,  under  the  existing  trade  con¬ 
ditions,  weather  their  clay.  The  face  brick  manufacturer,  therefore, 
must  allow  as  little  time  as  possible  to  elapse  from  the  time >  that  his 
clay  is  mined'  until  it  is  under  fire  in  the  kiln  if  he  wishes  to  avoid  that 
bane  of  the  face  brick  manufacturer,  scumming,  which  results  from  the 
formation  of  soluble  salts  by  the  sulphurous  and  sulphuric  acid  from 
iron  pyrites. 

To  the  front  brick  manufacturer,  the  presence  of  iron  pyrites  is  not, 
aside  from  the  question  of  scumming,  a  serious  disadvantage,  for  the 
black-slagged  specks  resulting  from  ferrous  iron  from  the  pyrites  fluxing 
with  the  silicates  is  not  objectionable  to  architects.  If,  however,  a  clean 
buff  brick  is  resired  or  if,  for  any  reason,  the  smoother  and  more  uni¬ 
formly  distributed  black  specking  by  the  use  of  pyrolusite  (MnO)  is 
needed,  then  a  clay  practically  free  from  iron  pyrites  must  be  used. 

In  face  brick,  soundness  and  color  are  the  prime  requisites.  In  pav¬ 
ing  brick,  toughness  alone  is  the  prime  requisite.  If  a  clay  contains  sul¬ 
phide  of  iron  (pyrites)  scattered  throughout  the  mass,  local  slagged 
spots  scattered  all  through  the  brick  will  be  formed  in  burning.  These 
slagged  spots  will  be  spongy  or  vesicular,  i.  e.,  full  of  cavities,  just  as 
is  the  black  warty  mass  that  appears  on  the  face  of  a  brick  made  from  a 
pyritiferous  clay.  The  local  fused  spots  are  detrimental  to  the  tough¬ 
ness  of  the  brick,  not  only  because  they  are  spongy  but  also  because  they 
<ire  fused  glassy  ferrous  silicates,  which  are  generally  very  brittle  and 
have  no  property  in  common  with  the  tough,  stony  matrix  which  makes 
up  the  body  of  the  brick. 


PUKDYj 


QUALITIES  OF  CLAYS  FOR  MAKING  PAVING  BRICK. 


227 


As  in  weathering,  the  first  step  in  the  oxidation  of  iron  pyrites  is  the 
separation  of  the  iron  and  sulphur.  In  the  kiln,  however,  sufficient 
length  of  time  cannot  be  allowed  to  drive  off  more  than  one  of  the  two 
atoms  of  sulphur.  The  first  atom  is  expelled  early  in  the  oxidation 
period  and  passes  off  in  the  waste  gases  as  sulphurous  and  sulphuric 
acid  gases.  The  remaining  atom  of  sulphur  is  not  expelled  readily,  in 
fact  either  a  very  long  time  or  much  higher  heat  is  required  for  its 
expulsion.  In  the  customary  heat  treatment  in  kilns,  this  last  atom  of 
sulphur  probably  remains  with  the  atom  of  iron  until  a  temperature 
is  reached  that  will  cause  fusion  between  the  iron  and  silicates,  form¬ 
ing  the  black  slag  mentioned  in  preceding  paragraphs.  It  can  be  said, 
therefore,  that  under  the  usual  kiln  treatment,  iron  pyrites  is  oxidized 
as  follows:  (1)  One  atom  of  sulphur  is  oxidized  and  expelled  during 
the  oxidation  period.  (2)  The  other  atom  of  sulphur  is  oxidized  and 
expelled  only  by  long  heat  treatment  or  higher  temperatures.  (3)  The 
atom  of  iron,  under  the  long  heat  treatment,  will  oxidize  to  the  higher 
oxide  forms  before  slagging  begins,  but  under  the  usual  heat  treatment, 
in  which  sufficient  time  is  not  allowed  for  the  oxidation  and  expulsion 
of  the  last  atom  of  sulphur  at  a  low  temperature  (about  500°C),  the 
iron  is  not  freed  from  its  sulphur  radical  and  oxidized  to  FeO  until  a 
temperature  has  been  attained  that  would  cause  this  FeO  to  flux  with 
silicates. 

From  these  discussions  of  oxidation  it  is  evident  that  a  good  paver 
cannot  be  made  from  a  pyritiferous  clay  unless  it  either  be  thoroughly 
weathered,  or  an  unusually  long  time  be  given  to  thoroughly  oxidize  the 
iron  and  sulphur  before  fusion  is  allowed  to  take  place. 

Other  Substances — There  are  many  substances  other  than  carbon  and 
iron  that  -suffer  oxidation,  but  inasmuch  as  their  oxidation  is  not  at¬ 
tended  with  serious  difficulties  and  is,  therefore,  of  little  consequence 
to  the  paving  brick  manufacturer,  they  will  not  be  discussed.  The 
gases  given  off  from  clay  wares  under  oxidizing  and  reducing  conditions 
are  just  now  being  studied  by  ceramists.  By  these  studies  many  phen¬ 
omena  in  fusion,  discoloration,  etc.,  of  pottery  wares  are  being  explained 
and  the  potter  is  receiving  much  benefit.  It  is  not  necessary,  at  this 
time,  to  discuss  the  results  of  these  studies. 

EFFECT  OF  PHYSICAL  AND  CHEMICAL  PROPERTIES  OF  CLAYS. 

The  effect  of  carbon  in  its  different  forms  has  been  discussed.  The 
oxidation  of  ferrous  compounds  in  the  presence  and  absence  of  carbon 
has  also  been  considered.  Certain  other  points  need  be  considered  un¬ 
der  this  heading.  Among  these  are:  distribution  of  the  carbon,  fine¬ 
ness  of  grain  of  the  clay,  iron  in  combination  as  a  stable  or  not  easily 
altered  silicate,  the  structure  of  the  clay  mass,  the  presence  of  moisture, 
and  the  temperature  factor. 

Varied  distribution  of  carbon — If  carbon  is  thoroughly  disseminated 
through  the  mass,  it  will  be  so  surrounded  by  the  mineral  matter  as 
to  cause  slow  oxidation  in  a  manner  similar  to  the  slow  burning  of  a 
fire  banked  with  earth.  If,  on  the  other  hand  the  carbon  is  concentrated 


228 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


in  particles  the  size  of  coal  dust,  oxidation  can  take  place  mnch  more 
readily.  Anthracite  screenings  added  to  a  clay  either  for  the  purpose 
of  effecting  equal  distribution  of  heat  or  making  the  ware  more  porus, 
seldom  give  trouble  in  oxidation.  The  quantity  of  carbon,  therefore,  is 
not  so  important  a  factor  in  determining  the  oxidizing  behavior  of  clay 
as  its  character  and  distribution.  On  this  account  chemical  analysis 
has  failed  to  give  much  aid'  in  detecting  the  difficultly  oxidizable  clays. 

Fineness  of  grain — If  the  clay  itself  is  fine  grained,  and  especially 
if  very  plastic,  it  will  prevent  oxygen  getting  to  the  carbon  and  will 
delay  the  expulsion  of  the  gases  formed  by  the  burning  carbon.  Fur¬ 
ther,  in  case  of  fine-grained  clays,  the  carbon  'will  not  be  completely 
oxidized,  i.  e.,  CO  instead  of  CO2  will  be  formed  and  in  escaping  from 
the  center  of  the  bricks  will  keep  the  iron  in  the  outside  portions  in  a 
reduced  condition  long  after  the  carbon  here  has  been  burned  out  and 
time  given  to  oxidize  the  iron  to  the  ferric  condition.  Fineness  of 
grain  of  a  clay,  therefore,  plays  an  important  role  in  the  oxidation  of 
clay  wares. 

Stable  iron  compounds — Iron  combines  with  silicates  in  both  the 
“ous”  and  “ic”  condition,  i.  e.,  we  have  ferrous  silicates  and  ferric 
silicates.  The  instances,  however,  of  iron  in  the  “ic”  condition  com¬ 
bining  with  silicates  are  comparatively  rare.  This  was  shown  in  a  very 
forcible  manner  in  a  series  of  experiments  in  which  the  writer  used 
several  varieties  of  granite  in  porcelain  floor  tile  bodies.  The  granites 
were  obtained  from  different  quarries  in  the  form  of  “spalls”  that  arc 
made  when  the  rough  granite  is  cut  into  shapes.  These  spalls  were 
ground  to  powder  that  was  as  fine  as  feldspar  and  flint,  as  prepared  by 
millers  for  pottery  use.  In  the  majority  of  cases  the  tiles  were  full  of 
minute  black  specks  with  but.  a  trace  of  the  buff  color  that  would  be 
given  if  the  iron  had  been  in  the  “ic”  condition.  In  one  or  two  cases 
the  iron  specks  were  buff  instead  of  black,  showing  that  the  iron  wa s- 
either  in  the  “ic”  form  in  the  granite  or  Had  been  oxidized  in  the 
burning.  From  the  fact  that  all  these  “granite  trials”  were  burned 
at  the  same  time  and  hence  under  the  same  heat  treatment,  it  was  con¬ 
cluded  that  in  these  exceptional  cases  the  iron  was  originally  in  the 
more  highly  oxidized  form.  This  conclusion  was  substantiated  in  later 
experiments  in  which  iron  calcines  were  used. 

When  fusion  in  a  clay  or  clay  mixture  has  progressed  sufficiently  to 
cause  the  whole  to  be  vitrified,  iron,  if  originally  present  as  an  oxide, 
carbonate  or  hydrate,  will  generally  combine  as  a  lower  oxide,  forming- 
ferrous  silicate.  The  blueing  of  fire  clays  and  the  changing  from  red 
to  chocolate  in  shales  is  evidence  of  this.  For  this  reason  iron  oxide 
added  to  a  porcelain  body  either  as  an  oxide  or  as  an  ingredient  of  a 
shale  will,  on  vitrification  of  the  body,  result  in  a  blue  tint.  Iron  pre¬ 
cipitated  into  a  mass  of  silica  or  alumina,  and  the  mixture  dried  and 
calcined  under  oxidizing  conditions,  will  when  added  to  the  porcelain 
body,  produce  a  buff  or  pink,  never  a  blue  color.  These  experiments 


PURDY]  QUALITIES  OF  CLAYS  FOR  MAKING  PAVING  BRICK.  229 

proved  conclusively  that  iron  can  combine  with  alumina  and  silica  in 
the  “ic”  condition  forming  ferric  compounds,  and,  further,  that  when 
so  combined  fusion  of  the  body  will  not  result  in  the  reduction  of  the 
iron  compound. 

The  practical  lessons  to  be  learned  from  these  two  experiments,  the 
first  with  granite  dust  and  the  second  with  iron  free  as  an  oxide  and 
combined  as  a  silicate,  are:  (1)  that  when  combined  as  a  ferrous  sili¬ 
cate  the  maintenance  of  strictly  oxidizing  conditions  in  a  kiln  will  not 
result  in  oxidation  of  the  iron;  (2)  that  iron  oxide  uncombined  is  not 
only  easily  reduced  but  will  form  ferrous  compounds  when  fused  with 
silicates;  (3)  that  when  iron  is  combined  as  a  ferric  compound  with 
alumina  and  silica,  it  will  retain  its  ferric  condition  against  the  re¬ 
ducing  influence  of  fusion  and  hence  is  very  apt  to  retain  its  “ic”  form 
even  under  reducing  conditions..  This  latter  statement  is  an  assump¬ 
tion,  for  no  direct  evidence  bearing  on  the  point  is  at  hand,  but  deduc¬ 
tion  from  known  data  seem  to  leave  no  doubt  as -to  the  validity  of  the 
assumption. 

In  clays  we  have  iron  combined  with  silicates  in  a  large  variety 
of  mineral  forms  and  compounds.  If  these  compounds  are  stable  when 
heated,  the  iron  will  retain  its  form  of  combination  against  oxidizing 
and  reducing  influences.  Iron  when  combined  as  a  silicate,  therefore, 
will  not  be  affected  during  the  oxidation  period.  In  this  connection, 
however,  chemical  analysis  of  a  given  clay  will  not  show  exactly  how 
much  of  the  iron  is  combined  or  in  what  form  it  is  combined  with  the 
silicates. 

Structure  of  clay  ware — Orton  and  Griffin  have  shown  that  the*  more 
porous  the  brick  the  more  readily  can  it  be  oxidized.  Soft-mud  bricks 
by  actual  porosity  determinations  were  found  to  be  the  most  porous, 
dry-press  somewhat  more  dense,  and  the  stiff-mud  bricks  the  most 
dense.  Our  experiments  have  shown  also,  that  clay  issuing  from  the 
machine  die  in  as  dry  or  "stiff”  a  condition  as  is  compatible  with  form¬ 
ation  of  a  perfect  bar,  will  produce  a  denser  brick  than  when  the  bar  is 
permitted  to  issue  in  a  softer  condition.  While  maximum  density  in 
unburned  bricks  means  minimum  toughness  that  can  be  produced  with  a 
particular  clay,  it  means  also  maximum  difficulty  in  oxidation.  This, 
however,  is  a  minor  factor  in  the  problem  of  oxidation  of  clay  wares, 
for  an  easily  oxidized  clay  will  still  be  easily  oxidized  and  a  difficultly 
oxidized  clay  will  be  difficultly  oxidized,  no  matter  how  dense  the  wares 
may  be  in  either  case. 

The  thickness  of  ware  and  consequently  the  manner  of  setting  is  a 
more  important  factor  than  density  of  the  clay  body.  Hollow  goods, 
where  the  walls  are  thin,  would  be  completely  oxidized  under  conditions 
that  would  not  permit  the  complete  oxidization  of  bricks  manufactured 
from  the  same  clay.  Depth  to  which  oxygen  must  penetrate  is  ob¬ 
viously  the  effective  factor  in  these  cases. 


230 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


Temperature  as  a  Factor  in  Oxidation — Quite  obviously,  the  higher 
the  temperature  the  more  rapid  will  be  the  combustion  of  the  carbon. 
In  the  case  of  clays  free  or  practically  free  from  iron,  or  where  the 
iron  is  in  a  stable  silicate  combination,  rapid  combustion  at  high  tem¬ 
peratures  would  have  no  attending  evil's  and  would  materially  shorten 
the  oxidation  period.  When  the  iron  is  present  in  the  “ous”  condition, 
or  where  it  can  be  easily  reduced  to  the  “ous”  form,  combustion  of  the 
carbon  at  temperatures  above  1000°  C.  would  result  in  partial  slagging 
of  the  iron  with  the  silicates  forming .  a  dark  gray  mass  that  cannot, 
without  expenditure  of  excessive  time,  be  reoxidized.  Such  action  would 
cause  premature  fusion  of  the  clay  mass,  especially  near  the  bag  walls 
of  the  kiln,  and,  as  a  consequence,  careening  of  the  whole  “setting, ”  or 
at  least  a  falling  over  and  fusing  together  of  the  bricks  near  the  bags. 

Oxidation  at  too  high  temperatures  is  frequently  shown  by  a  per¬ 
manently  discolored  center  or  core,  in  which  vitrification  has  progressed 
further  than  in  the  outside  shell  of  the  brick.  Orton  and  Griffin  found 
that  800°  C  was  the  safest  temperature  at  which  to  oxidize  the  average 
clay.  In  some  rare  cases,  like  the  clay  found  at  Loraine,  Ohio,  which 
Orton  and  Griffin  cited,  even  800°  C  would  be  too  high  for  safe  oxida¬ 
tion. 

Moisture  as  a  Factor  in  Delaying  Oxidation — In  the  majority  of  yards 
which  were  visited  by  the  writer  evidence  could  be  found  of  incomplete 
oxidation  of  a  few  bricks  in  an  otherwise  thoroughly  oxidized  kiln  of 
brick.  Inquiry  developed  the  fact  that  in  most  instances,  in  the  rush 
to  make  a  day’s  work,  the  setters  would  set  the  bricks  as  they  came 
from  the  dryer,  no  matter  how  wet  or  dry  they  may  have  been.  In¬ 
variably  either  the  head  setter  or  superintendent  would  recall  that  a 
carload  or  two  of  wet  bricks  were  set  in  the  particular  place  where  the 
unoxidized  bricks  were  found  on  “drawing  the  kiln.”  It  is  evident, 
therefore,  that  moisture  in  the  bricks  has  an  influence  on  oxidation  of 
•the  clay. 

Theoretical  calculations,  laboratory  experiments  and  factory  observa¬ 
tions  have  proved  that  wet  brick  set  in  a  kiln  of  dry  bricks,  are  de¬ 
layed  in  heating  up  by  the  fact  that  the  heat,  which  in  case  of  the  dry 
bricks  is  sufficient  to  carry  it  well  into  the  oxidizing  period,  is  spent  in 
evaporating  the  water  from  the  wet  bricks,  thus  delaying  their  “heat¬ 
ing  up”  process.  Bricks  thus  delayed  will  not  be  heated  much  more 
than  is  sufficient  to  cause  the  beginning  of  oxidation,  when  in  the  bulk 
of  the  bricks  oxidation  is  completed  and  fusion  begun.  Under  these 
conditions  the  bricks  that  were  wet .  will  pass  through  the  oxidizing 
period  (450  to  1000°  C)  too  rapidly  to  permit  their  complete  oxidation. 
Water,  therefore,  indirectly  delays  oxidation. 

PHYSICAL  AND  CHEMICAL  EFFECTS  OF  INCOMPLETE  OXIDATION. 

Usual  effects — Reduction  of  iron  and  the  consequent  early  fusion  of 
the  unoxidized  portion  of  a  brick  results  in  the  formation  of  a  par¬ 
tially  fused  glass  surrounded  by  a  shell  that  has  as  a  rule  just  begun  to 
vitrify.  Entrapped  in  this  glass  is  some  burned  carbon  which  when 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


281 


partially  oxidized  is  converted  into  a  gas.  Aside  from  the  CCb  formed 
by  the  oxidation  of  the  entrapped  carbon,  there  are  salts  that  are 
volatilized  into  vapors  at  this  heat.  These  gases  and  vapors  expand 
on  heating,  causing  the  black  unoxidized  core  of  the  brick  to  swell  up 
until,  in  Extreme  cases,  the  brick  is-  twice  its  normal  size  and  will  float 
in  water.  Inasmuch  as  the  oxidized  shell  is  thickest  on  the  edges  and 
thinnest  of  the  faces,  the  swelling  core  will  bulge  out  the  faces  of  the 
brick  until  it  approximates  the  shape  of  a  cylinder. 

It  is  obvious  at  once  that  bricks  which  have  swollen  centers  will  not 
be  fit  for  pavers.  It  follows  also  that  the  toughness  of  a  brick  is  lessened 
in  proportion  to  the  extent  that  its  center  is  reduced  and  rendered 
vesicular.  It  is  imperative,  therefore,  that  ample  time  be  given  at 
the  oxidizing  period  (red  heat)  to  insure  complete  combustion  of  the 
carbon  and  oxidation  of  the  iron. 

Exceptional  Effects — In  the  case  of  H,  23,  oxidation  had  not  pro¬ 
gressed  very  far  at  the  end  of  24  hours  exposure  at  650°,  and  the  un¬ 
oxidized  portion  of  the  briquettes  vitrified  on  further  heating  to  as 
hard  hard  and  dense  a  mass  as  did  the  outer  oxidized  portions.  Ho 
swelling  or  distortion  of  the  brick  due  to  the  oxidation  of  the  carbon 
and  ferrous  iron  was  noted.  In  fact,  the  shrinkage  and  rate  of  de¬ 
crease  in  porosity  was  not  abnormal  in  any  respect.  In  Fig.  25  are 
shown  the  volume-shrinkage,  porosity,  and  specific  gravity  curves  for 
this  clay. 

In  this  figure,  the  specific  gravity,  porosity  and  volume  of  the  bricks 
burned  at  different  temperatures  are  calculated  in  terms  of  the  per¬ 
centage  of  increase  or  decrease  over  those  of  the  unburned  bricks.  In 
other  w^ords,  the  raw  factors  are  considered  as  a  basis  from  which  the 
“burned”  factors  are  calculated  as  increase  or  decrease.  Zero,  there¬ 
fore,  represents  the  "data  obtained  from  the  unbumed  bricks. 

The  percentage  of  increase  of  the  burned  ware  over  that  of  the  un¬ 
burned  is  shown  above  the  datum  line  on  the  ordinate,  and  the  per 
centage  of  decrease  is  shown  below  the  datum  line.  On  the  abscissa 
is  shown  the  actual  percentage  of  porosity  of  the  burned  brick. 

Points  on  the  same  ordinate  represent  a  single  brick.  Data  from  all 
the  bricks  studied  in  this  test  have  not  been  plotted,  but  only  those  in 
which  the  percentage  of  porosity  differed  sufficiently  to  fix  points  on 
the  curves  that  would  show  a  comparative  increase  or  decrease  in  the 
several  factors. 

The  fact  that  the  actual  percentage .  of  porosity  of  the  burned  brick 
was  taken  in  each  case  as  a.  point  on  the  abscissa,  without  regard  to 
the  porosity  of  the  unburned  brick,  will  account  for  the  irregularity  in 
the  curves. 

notwithstanding  the  fact  that  the  black  unoxidized  core  remained, 
even  when  the  whole  exhibited  a  porosity  of  only  2  per  cent,  the  brick 
continued  to  shrink  normally  with  each  increase  of  temperature,  and 
the  specific  gravity  of  the  brick  decreased  less  than  in  the  case  of  many 


232 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


normally  burned  paving  brick  shales.  This  steady  decrease  in  volume 
and  comparatively  slight  increase  in  specific  gravity  gives  evidence  of  a 
thermo-physical  behavior  that  is  opposite  to  that  of  the  majority  of 
clays  containing  carbon. 


PERCENTAGE  OF  INCREASE  AND  DECREASE  FROM  INITIAL  CONDITION 


Fig.  25.  Physical  alterations  produced  by  burning  compared  with  unburned  con¬ 
dition  of  clay. 

Fusion1. 

FUSION  PERIOD  OF  CLAYS. 

From  the  laws  of  physical  chemistry,  it  could  not  be  expected  that 
the  heterogeneous  mineral  mass  called  clay,  consisting  largely  of  amor¬ 
phous  materials,  would  have  a  definite  fusion  point.  According  to 
Walker* 2,  this  would  more  properly  be  called  a  fusion  period. 

1A  large  part  of  this  discussion  of  the  fusion  period  appeared  by  permission  in 
advance  in  a  paper  by  Purdy  and  Moore  in  Trans.  Am.  Cer.  Soc.,  Vol.  IX. 

2  Introduction  to  Physical  Chemistry,  p.  6  4.  \ 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  HAVING  BRICK. 


233 


Our  studies,  a  part  of  the  data  of  which  are  shown  in  subsequent 
curves,  bear  out  this  statement.  It  will  be  seen  that  in  the  case  of  the 
purest  clays,  according  to  the  specific  gravity  curves,  fusion  begins  as 
early  as  cone  3.  In  the  case  of  some  of  the  most  impure  shales,  high 
in  lime,  fusion  begins  at  a  period  considerably  earlier  than  cone  010. 
Fusion  thus  early  begun  progresses  with  more  or  less  regularity  until 
the  whole  mass  enters  into  active  thermo-chemical  reaction1  and  de¬ 
formation  of  the  ware  ensues.  Incipient  vitrification,  vitrification,  and 
like  terms  are  only  descriptive  of  the  effects  at  different  stages  of 
fusion.  It  is  the  rate  of  fusion,  therefore,  that  determines  the  pyro- 
physical  effects  produced  -in  the  burning  of  clay  wares  during  this 
period. 

FACTORS  AFFECTING  RATE  OF  FUSION. 

Mineralogical  Composition — Synthetical  studies  of  the  fusion  of  mix¬ 
tures  of  pure  minerals,  have  shown  that  the  same  chemical  elements, 
brought  together  as  constituent  parts  of  different  minerals,  produce 
mixtures  having  unlike  fusion  periods.  The  rate  of-  fusion  and  the 
regularity  with  which  it  progresses,  as  well  as  the  point  of  complete 
yielding,  are  affected  very  largely  by  the  manner  in  which  the  various 
elements  are  previously  combined.  Because  of  the  difficulty  of  mak¬ 
ing  a  microscopic  mineralogical  analysis  of  a  clay,  we  are  not  able  to 
obtain  information  that  would  aid  in  an  attempt  to  foretell  or  explain 
in  full  the  fusing  behavior  of  clays.  Realization,  therefore,  of  the  fact 
that  difference  in  the  mineralogical  make-up  of  clays  of  like  ultimate 
chemical  constitution,  causes  difference  in  their  fusion  behavior,  is  the 
only  result  of  practical  value  that  has  so  far  come  from  the  study  of 
the  fusion  behavior  of  synthetical  mixtures  of  minerals. 

There  is  one  very  notable  exception  to  the  above,  and  that  is  in  the 
case  of  calcium  carbonate.  The  effect  of  calcium  carbonate,  depending 
upon  size  of  grain  and  extent  and  homogeneity  of  diffusion  throughout 
the  clay  mass,  operates  in  a  two-fold  manner.  If  thoroughly  blended 
with  the  clay  in  small  particles  a  portion  of  it  (on  the  average  up  to 
about  8  per  cent  of  the  total  clay  mass)  operates  as  a  very  active  flux. 
Its  fluxing  effect  is  most  notable  on  account  of  the  rapidity  with  which 
it  combines  with  clay  substance  to#form  a  molten  mass.  This  reaction 
is  in  some  instances  so  rapid  as  to  make '  it  very  dangerous  to  ap¬ 
proach  the  vitrification  temperature.  If  the  calcium  carbonate  is  pres¬ 
ent  in  nodules,  the  thermo-chemical  reaction  just  described  can  take 
place  only  at  the  points  of  contact  of  the  decarbonized  lime  and  clay, 

l  The  expression  “thermo-chemical  reaction”  is  used  here  because  we  are  accus¬ 
tomed  to  thinking  of  fusion  as  resulting  from  chemical  combination  of  the  clay- 
ingredients.  The  idea  conveyed  by  a  literal  definition  of  the  term  is,  however, 
very  erroneous.  The  alterations  in  the  clay  mass  during  fusion  are  more  largely 
that  of  mutual  solution  of  the  mineral  compounds  than  of  chemical  reaction  one 
with  another. 


234 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


the  remainder  of  the  carbonate  being  converted  into  quicklime.  The 
different  effects  of  lime  in  these  two  physical  conditions  one  the  rate 
and  regularity  of  fusion  of  the  clay  mass  is  obvious. 

In  the  very  valuable  researches  recently  published1  bv  Dr.  Reinhold 
Rieke,  it  is  demonstrated  that  lime  added  in  excess  of  a  given  amount 
does  not  act  as  a  flux  and  cause  sudden  failure  of  ware  with  slight  in¬ 
crease  of  heat  treatment.  In  fact,  it  happens  that  this  excess  lime  seems 
to  counteract  the  effect  produced  by  the  smaller  quantities.  Experi¬ 
ments  in  the  compounding  of  pottery  bodies  have  shown  that  notwith¬ 
standing  the  fact  that  wares  containing  lime  in  excess  of  this  amount 
do  not  fail  by  sudden  fusion,  i.  e.,  with  slight  increase  in  intensity  of 
heat  treatment,  they  suffered  a  rapid  decrease  in  porosity  and  specific 
gravity  when  the  heat  treatment  had  become  sufficiently  intense  to 
cause  the  formation  of  the  more  fusible  lime-silicate  solution.  It  ac¬ 
cords  entirely  with  the  facts  to  conceive  of  the  fused  portion  as  a 
mutual  solution  of  minerals  becoming  saturated  with  lime.  Up  to  the 
point  of  about  one-third  saturation  the  lime  is  very  active  as  a  flux  and 
decreases  in  activity  as  the  saturation  approaches  completion.  It  is 
easy  to  see,  therefore,  that  any  lime  which  may  be  present  in  quantities 
in  excess  of  that  which  can  go  into  solution  will  not  have  any  fluxing 
action. 

In  most  mineral  mixtures  (and  this  is  true  in  clays)  the  first  which 
fuses  is  not  the  most  fusible  individual  mineral  or  substance  which  may 
be  present.  The  first  to  fuse  will  be  the  most  fusible  mixture  of  the 
minerals  present  known  technically  as  a  eutectic  mixture.  This  mix¬ 
ture  may  consist  of  two  or  more  of  the  clay  ingredients.  Whatever  the 
mixture  may  be — and  this  depends  largely  upon  the  size  and  character 
of  the  grains — it  will  fuse  some  time  before  the  fusing  point  of  the 
most  fusible  mineral  has  been  reached.  This  is  shown  in  the  curves 
given  in  the  section  of  this  report  which  deals  with  the  chemical  prop¬ 
erties  of  clays. 

Now  (repeating  for  emphasis)  any  lime  in  excess  of  that  which  is 
required  to  form  this  most  easily  fusible  (eutectic)  mixture  which  is 
possible  with  the  kind  and  condition  of  minerals  present  in  a  given 
clay,  will  not  be  active  as  a  flux.  That  portion  of  the  lime  necessary 
to  form  this  eutectic  mixture  goes  into  solution  with  a  rapidity  which 
is.  inversely  as  the  degree  of  saturation.  The  lime  which  goes  into 
solution  is  least  active  as  a  flux  when  sufficient  is  present  to  completely 
saturate  the  fused  portion,  most  active  at  about  one-third  saturation. 

The  rate  of  formation  and  the  amount  of  fused  material  formed  in 
a  brick  very  obviously  determine  the  rate  at  which  the  open  pores  will 
be  eliminated.  Since  lime  readily  forms  solutions  with  silicates,  and 
particularly  with  clay  substance,  those  clays  which  contain  from  2  to  8 
per  cent  of  free  lime  will  vitrify  rapidly.  Other  clays  having  the  same 
ultimate  chemical  composition  as  the  rapidly  vitrifying  ones,  but  in 
which  the  lime  is  already  combined  as  in  a  lime-bearing  silicate,  will  not 
vitrify  rapidly,  other  factors  which  influence  fusion  being  equal.  We 


l  Sprechsaal,  Nos.  45  and  46,  Nov.  1907. 


PURDY] 


QUALITIES  OF  CLAYS  FOE  MAKING  PAVING  BRICK. 


285 


must  recognize,  therefore,  that  when  the  other  factors  which  effect  fusion 
are  the  same,  the  amount  of  lime  which  will  combine  to  form  this  most 
easily  fusible  mixture  depends  upon  whether  the  lime  is  free  or  com¬ 
bined,  as  well  as  upon  the  kind  and  relative  quantities  of  the  other 
oxides  present. 

-  •  The  per  cent  of  calcium  oxide  which  Rieke  found  would  form  the 
most  fusible  mixture  of  the  formula  XCaO  1  AhOs  vSiCb  were  as  fol¬ 
lows  : 

XCaO  1  A1,03  1  SiO,  ;  —  25.6  per  cent  CaO 

XCaO  1  A1,03  2  SiO,  33.4  per  cent  CaOi 

XCaO  1  AL03  S  SiO,  ;  —  33.1  per  cent  CaO 

XCaO  1  A1,03  4  SiO,  ;  —  24.6  per  cent  CaO 

I  In  each  of  these  mixtures  the  per  cent  of  calcium  oxide  taken  into 
solution  up  to  the  point  where  the  rate  of  solution  began  to  decrease 
as  shown  by  his  curves,  were  as  follows : 

XCaO  1  A1,03  1  SiO,  ;  —  7.9  per  cent  CaO 

XCaO  1  Al,Os  2  SiO,  ;  — 10.0  per  cent  CaOi 

XCaO  1  A1,03  3  SiO,  ;  —  9.8  per  cent  CaO 

XCaO  1  AL03  4  SiO,  ;  —  7.5  per  cent  CaO 

Size  of  Grain — The  full  significance  of  this  factor  can  be  appreciated 
'only  by  considering  extreme  cases,  as  in  the  case  of  calcium  carbonate, 
{above  cited,  or  as  in  a  mixture  of  two  minerals  such  as  feldspar  and 
l flint.  When  feldspar  and  flint  are  mixed  as  fine  powders  in  the  pro¬ 
portion  of  75  per  cent  feldspar  and  25  per  cent  flint,  the  mass  will  be 
gfused  to  a  fluid  at  approximately  1100°C  in  a  comparatively  short  time. 
If,  however,  these  two  minerals  were  placed  side  by  side  in  the  shape  of 
[rectangular  pieces  having  the  same  proportional  weight  as  in  the  first 
rcase,  the  only  fluxing  action  that  would  take  place  at  1100  °C  would  be 
'at  the  points  of  contact.  Even  if  the  heat  was  held  at  1100° C,  com¬ 
plete  fusion  of  the  two  pieces  of  mineral  could  only  take  place  when  the 
glass,  formed  at  the  point  of  contact,  enveloped  and  slowly  ate  into  the 
:  unfused  portions,  and  thus  produced  an  intimate  mixture  of  the  two 
:  minerals  by  diffusion  or  surface  tension.  It  is  common  experience  that 
if  complete  fusion  of  the  two  minerals  at  1100°C  is  desired  when 
brought  together  in  the  form  of  coarse  particles,  considerable  time  must 
►  be  allowed,  and  that  to  effect  complete  fusion  in  a  shorter  time,  the 
-heat  must  be  raised  from  1100°C  to  1230°C  (approximately),  or  the 
fusing  point  of  feldspar.  At  this  temperature  the  feldspar  melting 
would  completely  envelop  or  perhaps  float  the  flint  particles,  and  slowly 
attack  and  dissolve  them,  just  as  water  will  attack  and  dissolve  a  piece 
of  loaf  sugar. 

The  above  illustration,  while  an  exaggerated  case,  nevertheless  is 
descriptive  of  the  effect  of  fineness  of  grain  on  the  fusion  of  any  two 
minerals  which  the  mutually  soluble,  and  also  descriptive  of  the  fusion 
of  a  mixture  containing  particles  of  several  minerals,  as  a  clay. 

l  This  mixture  is  lime  with  pure  clay  substance.  Note  how  much  more  active 
the  lime  is  in  this  mixture  than  in  the  others. 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


236 


[BULL.  NO.  9 


In  the  burning  of  clay  wares,  where  time  is  an  important  and  un-  |. 
avoidable  factor,  the  effect  of  fineness  of  grain  influencing  the  fusing  S 
of  clays  is  particularly  noteworthy.  By  the  manufacturers  of  pyrome-  j 
trie  cones  it  has  been  recognized  as  such  a  powerful  factor  that  the  ut¬ 
most  care  is  taken  to  maintain  uniformity  in  size  of  grain  in  their  ; 
materials,  both  before  and  after  manufacture  into  powdered  cone  stock.  \\ 

The  statement  has  been  made  in  preceding  paragraphs  that  differ¬ 
ences  in  mineralogical  constitution  cause  differences  in  behavior  of  clays  j 
during  fusion.  That  statement  is  correct  for  the  heat  treatment  or  time  | 
and  temperature  required  to  affect  either  the  partial  or  complete  fusion  i 
of  the  mass.  It  would  not  be  correct,  as  will  be  shown,  if  the  tempera-  j 
ture  alone  was  considered. 

The  mixture  of  minerals  in  a  clay  which  has  been  ground  in  a  dry 
pan  is  far  from  being  homogeneous.  Our  discussion  earlier  in  this  I 
chapter  of  the  constitution  of  the  grains  should  make  it  plain  that  even  1 
if  they  were  as  finely  ground  and  as  thoroughly  disintegrated  as  is  prac-  t 
ticed  in  the  potteries,  the  mixture  would  lack  very  much  of  being  homo-  !j 
geneous.  Now  the  molten  silicates  are  so  viscous  that  diffusion  in  them  I 
is  exceedingly  slow  compared  with  diffusion  of  salts  in  water1  and  hence 
a  very  long  time  would  be  required  to  obtain  the  homogeneous  mixture 
that  is  necessary  before  the  mass  will  fuse  at  its  true  melting  point. 

Walker  has  been  quoted  to  the  effect  that  crystalline  substances  have  j 
a  definite  melting  point  while  amorphous  substances  do  not.  The  reason 
for  this  is  based  very  largely  upon  this  matter  of  absolutely  perfect 
homogeneity  of  constitution.  When  a  substance  crystallizes,  its  com¬ 
ponents  are  as  intimately  and  homogeneously  blended  as  it  is  possible 
to  conceive  of,  hence,  when  the  mass  fuses  the  components  are  in  a 
position  to  dissolve  in  one  another  as  soon  as  a  temperature  is  attained 
at  which  the  solution  is  affected.  In  amorphous  compounds2  we  have  j 
not  this  intimate  molecular  mixture3  and  hence  not  a  sharp  melting 
point.  In  the  case  of  clays  and  clay  mixtures,  where  we  are  not  able  to 
cause  a  mixture  of  the  components  that  is  any  other  than  a  compara-  ! 
tively  very  poor  approximation  to  intimacy  and  homogeneity,  it  must 
be  expected  that  'either  an  inordinarily  long  time  will  have  to  be  taken, 
or  a  temperature  higher  than  the  true  melting  point  of  the  mixture 
be  maintained  in  order  to  effect  the  fusion.  This  is  why  in  research 
laboratories  they  either  remelt  the  mixture  at  least  once  before  determ¬ 
ining  its  true  melting  point,  or,  take  it  to  complete  liquid  fusion  and 
note  the  temperature  at  which  the  mass  solidifies.  This  is  also  the 
reason  why  potters  find  that  a  mixture  will  melt  more  easily  the  second 
and  third  time.  This  is  also  one  of  the  reasons  why  so  much  stress  was 
laid  by  the  writer  upon  the  mineral  constitution  of  the  grains  as  Grout 
found  them  in  the  West  Virginia  clays. 

1  Diffusion  of  salt  in  water  is  so  slow  as  to  permit  of  easy  measurement.  Diffu¬ 
sion  of  sugar  in  a  cup  of  hot  coffee  is  so  slow  that  it  necessitates  stirring  in  order 
to  dissolve  a  teaspoon  full  of  sugar  within  a  reasonable  time. 

2  Here  reference  is  made  only  to  inorganic  compounds. 

3  This  is  not  the  sole  reason. 


PURDY] 


QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK. 


237 


S  Reference  was  made  to  the  attempt  by  Hoffman  and  Desmond  to  test 
the  refractoriness  of  clay  by  toning  np  or  down  as  the  case  required. 
The  only  reason  that  they  failed,  aside  from  the  fact  that  they  were  not 
taking  note  of  the  ultimate  composition  of  the  clays,  was  the  unequal 
degree  of  homogeneity  of  mixture  of  the  fusing  components.  Theii 
method  would  have  failed  even  had  the  clays  and  flux  been  ground  and 
mixed  as  thoroughly  as  is  possible  by  any  physical  means  so  far  devised. 
If  they  had  desired  to  be  extravagant  of  time  and  fuel  they  could  have 
caused  their  mixtures  to  fuse  at  the  arbitrarily  chosen  temperatures,  or 
even  lower.  They,  however,  were  not  seeking  to  determine  the  true 
melting  point  of  their  mixture  but  rather  its  refractoriness.  If  they 
had  been  seeking  the  true  melting  point  they  could  have  resorted  to  the 
Customary  method  of  noting  the  point  at  which  the  fused  mass  solidified. 

Refractoriness  of  a  clay  is  its  ability  to  withstand  heat  treatment. 
The  relation  between  refractoriness  and  true  melting  point  of  a  clay 
is  as  difficult  to  trace  as  the  relation  between  refractoriness  and  ultimate 
chemical  composition — if,  indeed,  it  is  not  more  difficult.  This  is  due 
principably  to  the  character  of  the  mineral  aggregate  contained  in  the 
clay. 

In  the  case  of  shales,  the  same  is  true  to  a  very  much  more  marked 
degree.  In  the  shales  the  rate  and  final  attainment  of  fusion  is  af- 
[fected  so  largely  by  the  character  of  the  mineral  aggregates  that  we 
find  clays  which  are  serviceable  for  paving  brick  manufacture  differing 
:very  greatly  in  physical  properties.  It  is  for  this  reason  in  large  part 
that  coarse-grained  clays  vitrify  more  closely  and  form  stronger  bricks. 
In  fact,  the  writer  does  not  know  of  a  single  factory  in  which  paving 
brick  is  manufactured  from  fine-grained  clays,  although  in  the  labora¬ 
tory  several  fine-grained  clays  have  given  promising  results.  If  there 
is  a  preponderance  of  stable,  not  easily  fusible  minerals  present,  there 
is  no  reason,  so  far  as  the  pyro-chemical  properties  are  concerned,  why 
fine-grained  clays  cannot  be  used  in  the  manufacture  of  paving  brick. 

Volatile  Matter-^ Chemically  combined  water,  carbonic  acid  gas,  car¬ 
bon,  etc.,  do  not  of  themselves,  on  expulsion,  cause  thermo-physical  and 
chemical  reactions  to  take  place  between  the  stable  bases,  acids  and  sili¬ 
cate  compounds  left  behind,  but  their  expulsion  does  involve  changes 
in  physical  and,  in  some  senses,  chemical  conditions  that  provoke 
thermal  reactions  between  the  remaining  substances.  For^  example,  in 
terra-cotta  lumber,  sawdust  is  added,  so  that  when  it  burns  out,  the 
mass  will  be  left  extremely  porous,  i.  e.,  not  dense,  as  it  would  otherwise 
have  been.  The  sawdust  in  this  instance  has  been  effective  in  opening 
the  structure  of  the  ware  and  preventing  the  particles  of  clay  from 
coming  within  fluxing  distance  of  one  another  as  they  otherwise  would. 
What  is  true  in  the  case  of  the  sawdust  in  terra-cotta  lumber  is  true 
of  combustible  organic  matter  in  clays.  It  is  obvious,  however,  that  the 
influence  of  carbnn  in  this  connection  depends  to  a  very  large  degree 
on  the  size  of  the  carbon  particles. 


238 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO. 


9 


The  effect  of  the  expulsion  of  CCfi  from  such  compounds  as  ferrous] 
carbonate,  calcium  carbonate,  etc.,  on  the  thermo-chemical  behavior  of 
clays,  is  another  familiar  phenomenon,  the  importance  of  which  is  not; 
recognized  in  the  attempt  to  interpret. the  results  of  an  ultimate  chem¬ 
ical  analysis.  If  two  equal  portions  of  the  same  clay  are  taken,  and  to 
the  one  a  quantity  of  red  iron  oxide  (FezCh),  while  to  the  other  am 
equivalent  quantity  of  powdered  ferrous  carbonate  (FeCCh)  is  added,  J 
and  the  two  mixtures  burned  under  the  same  thermal  conditions,  it  will : 
be  found  that  the  mixture  containing  the  ferrous  carbonate  will  begin ; 
to  fuse  earlier,  exhibit  a  more  erratic  rate  of  decrease  in  specific  gravity  1 
as  the  intensity  of  the  heat  increases,  and  may  or  may  not,  depending 
upon  conditions  other  than  those  here  considered,  cause  an  earlier  ulti- 
mate  fusion.  The  same  is  true  to  a  greater  or  less  extent  in  the  relative 
fluxing  effect  of  the  oxides  and  carbonates  of  other  bases.  The  same 
phenomena  are  also  notable  in  the  comparative  fluxing  effect  of  such  | 
hydrous  and  anhydrous  silicate  compounds  as  raw  and  calcined  kaolin. 

Meade1  and  Meller2  have  shown  that  mineral  mixtures  containing 
alkalies  lose  when  burned  as  high  -as  20  per  cent  of  the  total  alkalies  ! 
present.  Such  a  loss  is  bound  to  affect  the  fusibility  of  the  mass  very 
considerably.  Now  we  know  that  the  alkalies  are1  less  volatile  when  j 
combined  with  some  constituents  than  with  others.  The  amount  of  j! 
alkali  volatilized,  and  hence  the  effect  on  the  fusibility  of  the  clay,  is  j 
dependent,  therefore,  quite  largely  upon  the  manner  of  its  combination.  I 

Structure  of  Ware — Intimacy  of  contact  of  the  clay  grains  with  one  j 
another  is  probably  affected  more  largely  by  the  manner  in  which*  the 
mass  is  formed  into  ware  than  by  any  other  factor  within  the  power  of 
man  to  control,  save  the  grinding  of  the  clay.  In  dry-pressed  bricks  i 
the  clay  particles  are  not  in  such  close  contact  with  one  another  as  they 
would  be  if  the  ware  were  formed  by  the  stiff-mud  method.  In  soft-  I 
mud  bricks  the  excessive  amount  of  water  used  prevents  the  clay  parti-  j 
cles  from  coming  into  as  intimate  contact  with  one  another  as  in  the  stiff-  ; 
mud  manufacture.  As  a  result  of  these  differences  in  the  degree  of 
compactness  of  the  grains,  it  is  found  that  not  only  a  more  easily  vitri-  jl 
fled  and  fused  mass  is  formed,  but  also  that  the  resultant  ware  is  very 
much  stronger  when  made  by  stiff-mud  methods.  For  the  same  reason 
this  same  difference  is  found  between  the  pressed  and  jiggered  pottery 
wares. 

Material — Calcium  carbonate,  hydrates  of  silica,  alumina,  and  iron,  j 
as  well  as  zeolitic  compounds,  when  first  precipitated  or  formed,  are  in 
the  majority  of  cases  in  extremely  fine  grains.  The  fluxing  behavior 
of  any  substance  is  materially  different  when  thorough! v  disseminated 
in  minute  grains,  especially  in  the  colloidal  form,  thaff  when  present  in  !j 
coarser  grains.  Iron,  for  instance,  has  been  found  to  enter  into  chem¬ 
ical  combination  with  silica  as  a  ferric  silica  when  the  iron  is  precip¬ 
itated  on  flint  and  as  a  ferrous  silicate,  if  at  all,  when  the  two  are 
mixed  as  dry  powders.  The  vast  difference  between  the  fluxing  action 

1  “Portland  Cement,”  Easton,  Pa.,  1906. 

2  Trans.  Eng.  Cer.  Soc.,  Vol.  VI,  p.  130. 


\ 


purdy]  QUALITIES  OF  CLAYS  FOR  MAKING  PAYING  BRICK.  239 

of  ferrous  and  ferric  oxides  and  compounds  need  not  be  discussed  at 
this  time.  The  important  fact  in  this  connection,  is  that  it  depends  to 
a  very  large  extent  on  the  form  and  manner  in  which  the  iron  is  dis¬ 
seminated  through  the  clay,  as  to  whether  it  will  combine  as  the  lower 
or  higher  oxide.  What  is  true  of  iron  in.  this  respect  is  true  to  a  degree 
of  other  fluxes. 

Summary  of  Factors  Affecting  Manner  of  Fusion  of  Clays — First — 
The  manner  in  which  the  several  constitutent  elements  are  combined, 
one  with  another,  very  materially  affects  the  fluxing  behavior  of  a  clay. 

Second — The  size  of  grain  of  the  several  mineral  constitutents  is  an 
important  fa'ctor  in  determining  the  fusing  behavior  of  clays. 

Third — The  amount,  form,  and  character  of  the  volatile  constitutents 
of  clay  does  not  directly  affect  the  thermo-chemical  reactions,  but  the 
difference  in  physical  condition  and  structure  of  the  clay,  and  the  sta¬ 
bility  of  the  non-volatilized  compounds,  caused  by  the  expulsion  of 
these  substances,  does  materially  affect  the  manner  in  which  fusion 
takes  place. 

Fourth — The  importance  of  the  role  that  absorbed  salts  play  in  the 
fusing  behavior  of  clays  is  little  appreciated.  The  evidence  on  the 
manner  in  which  they  operate  is  so  indirect  that  definite  statements  or 
conclusions  are  impossible.  That  they  are  important  factors,  however, 
there  is  no  doubt. 

Fifth— Concerning  precipitated  materials,  we  have  evidence  from  syn¬ 
thetical  experiments  that  prove  beyond  doubt  that  they  must  be  con¬ 
sidered  as  most  potent  in  affecting  the  fusion  of  clays. 

From  the  above,  it  is  evident  that  the  writer  has  but  little  confidence 
in  the  efficiency  of  an  ultimate  analysis  of  a  clay  as  a  means  of  fore¬ 
telling  its  burning  properties.  The  combination,  size  and  character  of 
grain,  solubility,  volatility,  and  dissemination  of  the  several  salts,  and, 
lastly,  the  manner  in  which  the  uncombined  oxides  are  introduced  into 
the  clay  are  more  effective  factors  than  the  total  ultimate  composition. 

Relation  of  Chemical  and  Physical  A)XSt>tution  to  Behavior 

in  Fusion( 

CHEMICAL  COMPOSITION. 

Historical — Search  in  ceramic  literature  disclosed  the  fact  that  prac¬ 
tically  no  data  have  yet  been  published  that  have  a  direct  bearing  on  the 
relation  of  chemical  and  physical  constitution,  behavior  of  clays  in 
fusion,  and  toughness  of  the  burned  ware.  Ogden1  did  some  prelimin¬ 
ary  work  on  the  relation  of  composition  to  toughness  in  porcelains  and 
found  the  remarkable  fact  that  increase  of  clay  content  from  30  to 
60  per  cent  caused  a  decrease  in  the  toughness  of  porcelain.  Inasmuch 
as  he  employed  the  “rattler  test”  in  determining  relative  toughness  of 


l  Trans.  Am.  Cer.  Soc.,  Vol.  VII. 


240 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


his  bodies,  his  studies  are  directly  applicable  to  the.  study  of  paving 
brick  clays.  While  the  development  of  toughness  has  not  been  shown 
to  have  a  direct  relation  to  the  rate  and  manner  of  vitrification  except 
in  our  own  results,  yet  that  such  a  relation  exists  can  be  assumed  until 
other  evidence  proves  the  contrary.  If  this  assumption  is  correct,  Og¬ 
den’s  results  would  show  that  the  evidence  developed  by  metallurgists 
to  the  effect  that  addition  of  either  aluminum  oxide  or  silicon  oxide  not 
only  raises  in  degrees  centigrade  the  period  at  which  fusion  is  com¬ 
pleted,  but  also  increases  the  viscosity  of  the  molten  mass,  and  the  rate 
at  which  verification  takes  place,  is  not  applicable  to  certain  mixtures 
It  must  be  admitted  that  before  Ogden  published  his  results,  ceramists 
entertained  the  belief  that  the  greater  the  content  of  AhOs  and  SiO* 
in  clays,  the  greater  would  be  the  toughness.  The  findings  in  the  case, 
of  fire  clays  here  reported  confirm  Ogden’s  ideas. 

In  the  following  paragraphs  will  be  given  such  evidence  as  seems  to 
bear  on  this  point. 

Effect  of  AW*  in  Ceramic  Mixtures — It  has  been  known  for  some 
time  that  the  addition  of  AhOs  to  clays  and  clay  mixtures  increases  their 
refractoriness1.  Fire  clays,  high  in  AhOs,  are,  as  a  rule,  the  most  re¬ 
fractory.  AhOs  not  only  raises  the  actual  period  at  which  fusion  is 
completed  but  also  causes  the  wares  made  from  aluminous  clays  to 
soften  and  deform  very  slowly.  The  slower  softening  and  deformation 
of  ware  made  from  aluminous  clays  has  been  attributed  to  increase  of 
viscosity1  of  the  mass  caused  by  alumina. 

The  writer  has  shown2  that  the  addition  of  AhOs  as  a  constituent  of 
clay  to  stoneware  glazes  until  the  proportion  of  alkali  and  alkaline  earth 
to  alumina  was  2.5  to  1,  not  only  rendered  the  glaze  more  fusible  but 
also  less  viscous.  Additions  of  AhOs  above  this  proportional  amount  in¬ 
creased  the  refractoriness  of  the  glaze,  if  not  its  viscosity.  Addition  of 
AhOs  as  a  constituent  of  feldspar  did  not  have  as  great  effect  on  the 
fusibility  of  the  glaze  as  did  the  same  equivalent  of  AhOs  from  clay, 
notwithstanding  the  additional  alkali  that  would  be  introduced  by  the 
feldspar. 

From  these  stoneware  glaze  studies  it  was  concluded  that  it  was  not 
so  much  a  question  of  quantity  of  AhOs,  but  of  the  manner  in  which  it 
was  added.  If  added  as  a  constitutent  of  clay  it  is  already  combined 
with  silica  and  water.  Whether  it  is  this  mutual  solution  of  calcium 
carbonate  and  clay  that  caused  greater  ultimate  fusibility  in  the  stone¬ 
ware  glazes,  when  clay  was  increased  to  a  definite  amount,  or  whether 
it  was  a  complex  case  of  an  eutectic  mixture  of  several  substances,  is 
not  yet  determined.  The  fact  remains  that  additon  of  clay  did  cause 
greater  fusibility  and  less  viscosity,  notwithstanding  the  fact  that  with 
each  addition  of  clay  the  AhOs  was  being  increased. 

Bleininger3  has  shown  experimentally  that  calcium  carbonate  reacts 
with  finely  pulverized  feldspar  as  readily  as  with  washed  kaolin.  From 
his  results  it  would  seem  as  though  fusion  is  initiated  between  calcium 

1  Molasses  is  more  viscous  than  water,  i.  e.,  it  flows  more  sluggishly.  Its  mole¬ 
cules  are  less  free  to  move.  Slow-flowing  fluids  are  said  to  be  viscous. 

2  Trans.  Am.  Cer.  Soc.,  Vol.  V. 

3  Geol.  Surv.  of  Ohio,  Bull.  No.  3,  4th  series,  p.  128. 


PURIFY]  PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES.  241 

carbonate  and  feldspar  as  early  as  between  calcium  carbonate  and  kaolin 
(pure  clay).  This  being  the  case  it  would  seem  as  though  the  addition 
of  •  clay  to  stoneware  glaze  mixtures  was  merely  the  formation  of  a 
eutectic  mixture  of  minerals.1 

Evidence  thus  far  developed  in  the  case  of  simple  mixtures  is  sum¬ 
marized  in  the  following  table : 

TABLE  XXXI. 

Showing  the  proportions  by  weight,  which  cause  maximum  fusibility  between 
the  two  mineral  substances  stated  in  each  case. 

(1)  (2) 

(1)  Magnesium  carbonate  (1)  and  kaolin  (2)  . 2  3 

Calcium  carbonate  (1)  and  kaolin  (2)  . 2  3 

Finely  pulverized  flint  (1)  and  kaolin  (2)  . 2  7 

Finely  pulverized  flint  (1)  and  feldspar  (2)  . 1  3 

(1)  With  quick  fire. 

Any  increase  or  decrease  of  AkCk  outside  of  the  limits  given  in  the 
above  table  results  in  increase  of  refractoriness  of  the  mixtures  as 
shown  in  the  several  curves  to  which  reference  has  been  made.  Similar 
points  of  greatest  fusibility  hawe  been  noted  in  the  case  of  glazes,  but 
data  have  not  been  obtained  that  permit  showing  the  facts  in  tabular 
or  curve  form.  AhO  then  increases  the  fusibility  of  mineral  mixtures 
when  added  in  amounts  not  exceeding  a  given  proportional  limit,  the 
limit  being  different  for  different  mixtures. 

Second,  in  slags,  glazes  and  glasses  addition  of  ALOs  above  a  given 
amount  increases  their  viscosity,  but  no  limiting  points  have,  as  yet,  been 
determined  except  in  the  case  of  slags.  Since  slags  are  comparatively 
simple  in  composition  and  usually  relatively  high  in  lime,  we  can  learn 
very  little  by  reviewing  in  detail  the  researches  that  have  been  made 
on  the  vicosity. 

Third — Increase  of  AbO  in  small  amount  in  glasses  increases  their 
toughness.  So  far  as  data  have  been  obtained  increase  of  AhCh  in  por¬ 
celain  bodies  does  not  increase  their  toughness. 

From  these  conclusions  a  query  is  at  once  presented  concerning  the 
relation  between  fusibility,  viscosity  and  toughness.  At  present  any  dis¬ 
cussion  of  this  query  would  be  based  wholly  on  assumption,  for  there 
are  no  experimental  data  bearing  on  the  point. 

Effect  of  Silica  in  Ceramic  Mixtures — Anhydrous  silica  is  practically 
inert  at  ordinary  temperatures,  but  at  the  temperature  usually  attained 
in  brick  kilns  it  becomes  very  active,  forming  compounds  having  very 
varied  oxygen  ratios,  i.  e.,  amount  of  oxygen  in  the  basic  to  the  oxygen 
in  the  acid  oxides. 

On  heating,  silica  expands  considerably,  indicating  peculiar  molecu¬ 
lar  changes.  LeChatelier2  has  shown  that  at  500 °C.  this  molecular 

1  The  mixtures  that  gave  the  greatest  fusibility,  as  shown  in  each  of  the 
figures  19,  20  and  21,  are  said  to  be  eutectic  mixtures. 

2  See  Bleininger,  Ohio  Geol.  Surv.,  Bull.  3,  p.  28. 


—16  Q 


242  PAVING  BRICK  AND  PAVING  BRICK  CLAYS.  [bull.  no.  9 

change  takes  place  to  a  very  pronounced  degree  in  all  forms  of  silica, 
the  least  in  amorphous  and  the  most  in  highly  calcined  flint.  Per¬ 
manent  expansion  in  highly  silicious  bricks  and  the  “punkness”  of  bricks 
made  from  a  mixture  of  clay  and  sand  are  evidence  of  the  effect  of  this 
peculiar  property  of  silica. 

No  matter  how  fine  the  free  silica  is,  it  does  not  seem  to  be  as  active  in 
forming  new  silicate  compounds  under  the  influence  of  heat  as  is  the 
silica  that  is  previously  combined,  as  for  illustration,  in  clay  or  feldspar. 
In  other  words,  silicate  combination  with  free  basic  elements  is  affected 
more  readily  when  the  silica  is  added  to  the  mixture  as  a  constituent 
of  a  pre-existing  silicate.  This  was  shown  very  prettily  in  an  experi¬ 
ment  reported  by  Bleininger.1  He  prepared  a  mixture  of  20  per  cent 
finely  ground  flint  and  80  per  cent  precipitated  calcium  carbonate  and 
two  other  mixtures  each  containing  respectively  20  per  cent  finely 
ground  feldspar  and  20  per  cent  of  kaolin  with  80  per  cent  calcium  car¬ 
bonate.  These  mixtures  were  maintained  at  a  temperature  of  1100  C. 
for  75  minutes.  At  this  temperature  calcium  silicate  compounds  are 
formed  which  are  soluble  in  hot  hydrochloric  acid  and  sodium  carbonate 
solutions.  The  residue  left  after  this  acid  and  alkali  treatment  is  the 
material  which  is  unattacked  or  unlocked  by  the  fluxing  action  of  the- 
lime.  In  the  following  table  are  Bleininger’s  results. 

Table  XXXII. 


Ground 

Flint. 

Ground 

Feldspar. 

Ground 

Kaolin. 

Per  cent  residue . 

28.83 

3.75 

3.07 

Per  cent  taken  into  solution . 

71.17 

96.25 

96.93  * 

Bleininger’s  results  strongly  support  the  doctrine  that  has,  for  the 
sake  of  .emphasis,  been  repeatedly  stated  in  this  report,  to-wit :  That 
very  little  can  be  told  concerning  the  fusing  behavior  of  silicate  mix¬ 
tures  from  an  ultimate  analysis,  for  if  this  were  not  the  case,  feldspar 
should  have  reacted  far  more  vigorously  with  calcium  carbonate  than 
did  clay.  Since  cement  investigators  have  found  that  the  hydrous  am¬ 
orphous  silica  reacts  with  lime  in  a  manner  similar  to  finely  pulverized 
crystalline  quartz,  it  can  be  readily  seen  that  misleading  data  would  be 
obtained  even  in  the  rational  analysis,  in  which  the  hydrous  amorphous 
silica  is  taken  into  solution  by  the  sulphuric  acid  and  thus  considered  as 
a  part  of  the  clay  substance. 

Addition  of  silica  to  pure  clays  like  shales  increases  their  refractori¬ 
ness  and,  (reasoning  from  data  on  slags)  possibly,  their  viscosity.  There 
is  no  evidence  showing  that  the  addition  of  flint  to  a  clay  increases  its 
toughness,  but  quite  the  contrary,  empirical  experiments  by  several  prac¬ 
tical  brick  manufacturers  have  proved  that  the  additon  of  ordinary  bank 
sand  makes  the  bricks  less  tough  or  even  very .  “punky.”  On  the  other 
hand  an  investigation  by  Worcester2  proved  that  Bedford  shale,  which 


lLoc.  cit..  p.  128. 

2  Trans.  Am.  Cer.  Soc.  Vol.  II,  pp.  295. 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


243 


outcrops  near  Columbus,  0.,  is  materially  benefited  by  an  addition  of 
crushed  Berea  sandstone  from  the  same  locality.  Instances  are  recorded 
lof  addition  of  certain  sands  in  Europe  having  proved  beneficial,  but  in 
neither  Worcester’s  experiments  nor  in  the  European  cases  was  there 
‘reported  a  determination  of  the  effect  of  sand  on  the  toughness  of  the 
-  burned  mixtures. 

In  the  manufacture  of  floor  tile  the  writer  found  that  a  porcelain 
.  body  consisting  of  40  per  cent  clay,  45  per  cent  feldspar  and  15  per  cent 
flint  was  much  tougher  than  a  body  containing  35  per  cent  clay  and  65 
per  cent  feldspar.  It  is  impossible  to  say  why  the  body  containing  flint 
should  be  tougher  but  certainly  some  credit  must  be  given  to  the  influ¬ 
ence  of  the  flint. 

Reviewing  the  known  facts  about  the  effect  of  silica  on  either  the 
fusion  of  clays  or  development  of  toughness  in  clay  wares,  it  must  be 
admitted  that  we  have  not  at  present  much  positive  evidence. 

Effect  of  Magnesium  Oxide  in  Ceramic  Mixtures — In  figure  20  on 
page  209  is  shown  graphically  in  fluxing  effect  of  magnesium  oxide 
with  kaolin.  Metallurgists  report  that  magnesium  oxide  is  a  much 
“harder”  flux  than  calcium  oxide  and  produces  a  much  more  viscous 
slag.  Ceramic  investigators  have  reported  conflicting  results  in  their 
attempts  to  use  MgO  as  a  flux,  some  claiming  that  it  is  more  active  than 
CaO  and  some  that  it  is  less  active.  Claims  have  been  made  by  some 
that  in  glazes  it  gives  greater  fusibility  and  slower  fusion,  while  others 
claim  opposite  results.  From  this  accumulation  of  apparently  conflict¬ 
ing  data  it  has  been  shown  that  in  short  quick  burns,  as  in  experimental 
kilns,  MgO  is  an  active  flux  causing  more  rapid  fusion,  but  in  longer 
burns  its  fluxing  action  begins  as  early  as  in  the  shorter  burns  but  pro¬ 
gresses  less  rapidly  and  requires  more  intense  heat  treatment  to  effect 
complete  fusion. 

•  The  lag  in  the  fusion  of  mixtures  containing  magnesium  oxide  is  at¬ 
tributed  to  either  the  viscosity  of  the  resulting  magnesium  silicate,  if 
it  enters  into  combination  with  the  glassy  matrix  that  fills  and  seals  the 
pores  of  vitrifying  wares,  or.  to  the  formation  of  non-fluid  magnesium 
compounds.1  Cement  investigators  claim  that  the  alkaline  earth  sili¬ 
cates  formed  by  heating  mixtures  of  clay  and  calcium  or  magnesium  car¬ 
bonate  at  temperatures  below  that  required  to  cause  sintering  of  the 
mass  into  a  hard  cake  or  brick  are  simple  silicates  of  calcium  or  mag¬ 
nesium  oxide  which  are  not  necessarily  fluid.  At  any  rate,  the  effect 
of  magnesium  in  ceramic  mixtures  differs  from  that  of  calcium  in  that 
the  magnesium  mixtures  fuse  very  slowly  over  a  long  heat  range,  while 
the  calcium  mixtures,  especially  when  present  in  amounts  equal  to  or 
more  than  10  per  cent,  remain  porous  up  to  the  time  that  fusion  begins, 
and  then  fluxing  ensues  very  rapidly  causing  the. ware  to  pass  from  por¬ 
ous  into  the  overburned  condition  within  a  very  short  range  of-  heat 
treatment. 

l  Eckel  states  in  “Cements,  Limes  and  Plasters,”  p.  154,  that  when  magnesia 
is  burned  in  a  quick  fire  its  density  (specific  gravity)  is  3.0  to  3.07,  while  if 
burned  in  a  slow,  long-continued  fire  its  specific  gravity  will  range  from  3.6  to  3.8. 


244 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[bull.  no.  9 


The  only  known  facts  concerning  the  influence  of  magnesium  oxide  in 
ceramic  mixtures  are  :  (1)  magnesium  oxide  increases  viscosity;  (2)  mag¬ 
nesium  oxide  causes  slower  rate  of  fusion,  at  least  when  it  is  the  pre¬ 
dominating  flux;  (3)  as  has  been  stated  earlier  in  this  report,  clays 
which  make  good  paving  bricks  contain  a  larger  amount  of  magnesium 
than  calcium  oxide;  (4)  the  Italians  are  now  making  low-fired  porce¬ 
lain  of  which  toughness  is  a  special  feature,  and  in  which  magnesium  is 
the  only  Ro  or  fluxing  base  present. 

Effect  of  Calcium  Oxide  in  Ceramic  Mixtures — Watts1  has  shown  that 
the  presence  of  a  small  amount  of  calcium  oxide  in  porcelain  mixtures 
results  in  increased  toughness  of  the  ware.  His  investigations  are  not, 
however,  sufficiently  exhaustive  to  warrant  more  definite  statement. 

It  is  known  that  lime  causes  a  breaking  down  of  the  silicates  with 
comparatively  little  heat  treatment,  and  also  that  the  new  silicates 
formed  are  probably  very  simple  in  composition  until  higher  tempera¬ 
tures  are  attained,  in  which  event  these  simple  silicates  suddenly  fuse, 
causing  the  whole  to  pass  rapidly  into  a  fluid  mass. 

Dr.  Rieke2  has  shown  that  in  mixtures  of  from  1  to  10  per  cent  of 
calcium  carbonate  with  kaolin  very  close  tight  bodies  are  obtained  which 
have  quite  a  large  range  of  vitrification  and  in  the  end  fuse  quite 
gradually.  Mixtures  containing  more  than  10  per  cent  of  calcium  car¬ 
bonate  remain  quite  open  until  final  fusion  begins,  at  which  time  the 
whole  mass  fuses  very  rapidly. 

In  comparison  with  Rieke’s  work  it  is  of  interest  to  study  results 
obtained  by  Nauss,3  who  worked  with  a  mixture  similar  to  Rieke’s  high 
calcium  body. 

The  two  bodies  were  as  follows : 

Table  XXXIII. 


Nauss. 

Rieke. 

Calcium  carbonate . 

70 

70 

Kaolin .  . 

11.05 

30 

Flint  : . 

18.66 

In  the  following  table  are  Nauss*  results  and  in  a  separate  column 
are  placed  the  data  obtained  by  Rieke.  Rieke  measured  his  heat  by  cones 
and  hence  the  temperatures  obtained  in  these  two  studies  cannot  be  com¬ 
pared  closely.  Since,  however,  Rieke  used  a  Seger  trial  kiln  and  very 
short  firing  periods,  his  cone  readings  can  be.  approximated  in  terms 
of  degrees  centigrade  within  the  accuracy  of  and  discrepancy  between 
the  method  by  which  each  research  was  executed. 

1  Trans.  Am.  C'er.  Soc.,  Vol.  V,  pp.  175. 

2  Sprechsaal  No.  38,  1906. 

3  Reported  by  Bleining’er,  Ohio  Geol.  Surv.  Bull.  No.  3,  p.  175. 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


245 


Table  XXXIY. 

Reaction  of  Calcium  Oxide  upon  Kaolin  and  Quartz. 


o 


1 

2 

3 

4 

5 

6 

7 

8 
9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 
21 
22 

23 

24 

25 


Temp.  °C . 

Loss  on  burning.... 

Loss  on  completed 

ignition . 

Carbon  dioxide 

remaining . 

Fusible  silica— per 

cent . 

Remarks. 

Rieke’s 

Results. 

*T3 

o 

o 

CO 

•< 

O 

ero 

CD  D 

P  0! 

S' 2, 

crq 

550 

585 

610 

655 

700 

725 

750 

775 

800 

850 

900 

950 

1000 

1050 

1100 

1150 

1150 

1200 

1200 

1250 

1250 

1300 

1300 

1325 

1.79 
1.88 

1.80 
1.91 
4.81 
6.18 
6.99 
8.35 

12.51 

20.34 

24.42 

27.56 

30.00 

29.75 

29.65 

29.74 

30.10 

30.10 

30.10 

30.10 

30.10 

30.10 

30.10 

30.10 

27.79 

27.96 

28.67 

28.83 

25.17 

24.30 

23.02 

21.90 

17.94 

10.64 

5.89 

2.88 

0.60 

28.48 

28.44 

28.84 

27.83 

25.89 

24.58 

23.88 

21.07 

18.72 

7.32 

5.29 

2.17 

0.40 

. 

19.70 

19.00 

18.90 

18.83 

18.75 

18.75 

18.26 

16.19 

16.10 

15.37 

13.13 

13.34 

11.97 

10.68 

30.3 

05 

Held  at  this  temperature  for  12  hours. . 

42.5 

'  2 

45.9 

5 

Dusted,  i.  e.,  fell  to  pieces . 

Dusted . 

Dusted . 

42.0 

8 

Began  to  fuse,  “melt” . 

10.2 
Fused . 

10 

23-24 

Professor  BleiningePs  conclusions  from  Nanss*  work  are: 

“First — In  regard  to  the  decomposition  of  calcium  carbonate,  it  is  clearly 
shown  that  it  begins  to  break  up  between  610°  and  650°C.,  and  before  700° 
is  reached  the  evolution  of  carbon  deoxide  is  going  on  quite  rapidly.  At 
1000°  the  evolution  is  practically  at  an  end.” 

“Second — On  examining  the  amounts  of  insoluble*  residue  and  comparing 
the  percentage  with  the  known  amount  of  quartz  in  the  mixture,  18.66  per 
cent,  and  making  allowance  for  the  small  amount  of  quartz  in  the  kaolin 
itself,  it  is  seen  that  the  kaolin  is  decomposed  completely  at  850°C.,  and  al¬ 
most  completely  at  800°C.” 

*  Third — Free  quartz  seems  to  be  attacked  by  the  calcium  oxide  soon  after 
the  completion  of  the  decomposition  of  kaolin,  probably  at  about  950°C., 
which  reaction  continues,  at  an  increasing  rate  up  to  the  highest  tempera¬ 
ture  employed  in  these  experiments.  It  is  quite  evident,  also,  that  the  length 
of  time  of  burning  influences  the  amount  of  quartz  attacked  somewhat,  so 
that  by  longer  burning,  at  least  with  temperature  over  1100°,  more  quartz 
may  be  rendered  soluble  than  in  a  short  period  of  ignition.” 

Prof.  Bleininger,  continuing,  says: 

“A  very  interesting  fact  was  brought  out  by  the  tendency  to  dust  observed 
with  the  mixture  at  temperatures  above  1200°C.  While  at  1200°  the  bri¬ 
quettes  were  hard,  at  1250°  they  dusted  very  rapidly,  and  at  1300°  almost 
instantaneously.” 


l  Insoluble  in  hydrochloric  acid  and  sodium  carbonate  solutions. 


246  PAVING  BRICK  AND  PAVING  BRICK  CLAYS.  [bull.  no.  9 

“On  calculating  the  formula  of  this  mixture  from  the  composition  we  find 
it  to  be  1.77  CaO,  0.108  A1203,  Si02,  that  is  not  quite  a  singulo  calcium  sili¬ 
cate,  and  hence  must  properly  be  classed  within  the  group  of  natural  ce¬ 
ments.  It  is  not  difficult  to  understand  that  the  dusting  must  be  coincident 
with  a  significant  molecular  change  from  the  condition  of  the.  loose,  friable 
mixture  to  a  hard  body  breaking  down  at  once  to  a  powder.  Might  not  this 
fact  indicate  that  up  to  1200°  these  calcareous  mixtures  are  but  pozzuolane- 
like,  simple  silicates,  consisting  of  silica  and  base  which  on  further  applicar 
tion  of  heat  become  chemically  more  complex  and  non-or  but  slightly  hy¬ 
draulic?  This  view  is  strengthened  „by  the  results  of  another  investigation 
which  have  shown  that  on  increasing  the  free  silica,  with  but  sufficient  base 
to  convert  the  quartz  into  the  active  state,  the  hydraulicity  is  practically  as 
great  as  with  a  greater  amount  of  base.”i 

Rieke’s  data  is  evidence  that  Bleininger’s  query  can  be  answered  in 
the  affirmative,  for  it  was  at  this  same  temperature,  1200 °C.,  that  his 
body  ceased  to  increase  in  porosity  and  began  to  vitrify.  From  1200°  C. 
on,  Rieke’s  body  vitrified  quite  rapidly  showing  that  “a  significant 
molecular  change”  is  taking  place.  From  Nauss’  results  it  must  be 
conceded  that  the  clay  has  suffered  a  very  significant  change.  NcJ 
doubt  it  has  passed  completely  into  solution  with  lime  and  silica.  In 
fact  Bleininger’s  results  given  in  Table  XXXYI  page  248  proves  this 
to  be  so. 

Rieke’s  porosity  data  show  also  that  prior  to  this  critical  tempera¬ 
ture,  1200°,  (rough  approximate)  the  grains  must  be  changing  form 
and  size,  for  the  mass  is  getting  more  porous  with  each  increase  in  heat 
treatment,  yet,  according  to  his  shrinkage  data  (1.2  per  cent  at  cone 
05  and  3.7  per  cent  at  cone  5)  the  mass  as  a  whole  is  decreasing  in  vol¬ 
ume.  Similar  simultaneous  increase  in  porosity  and  decrease  in  volume 
was  noted  in  several  instances  in  our  own  researches,  so  this  phenom¬ 
enon  is  not  alone  peculiar  to  simple  mixtures  high  in  lime. 

Important  as  are  these  observations,  and  especially  that  of  complete 
solution,  and  possibly  the  formation  of  entirely  new  compounds  before 
the  mass  begins  to  decrease  in  porosity,  i.  e.,  vitrify,  the  more  important 
item  to  note  at  this  time  is,  in  the  writer’s  opinion,  the  difference  in 
the  ultimate’  fusion  behavior  of  the  two  bodies,  the  one  containing  fred 
silica  and  the  other  supposedly  none.  It  was  shown  by  Bleininger’s 
result1 2  that  quartz  is  not  nearly  as  readily  attacked  by  CaO  as  is  kaolin 
or  feldspar,  and  hence  it  could  be  inferred  that  the  higher  the  content 
of  quartz  in  a  mixture,  the  later  and  slower  would  the  mass  fuse.  In 
decided  contradiction  to  such  an  inference  we  find  that  in  Xauss’  body, 
containing  18.7  per  cent  quartz,  the  original  minerals  have  been  com¬ 
pletely  broken  down  and  the  whole  began  to  “melt”  at  the  same  tem¬ 
perature  at  which  Rieke’s  body  containing  no  quartz  exhibits  a  porosity 
of  10  per  cent,  but  complete  fusion  does  not  take  place  until  a  tem¬ 
perature  of  about  1600°  C.  has  been  reached.  We  are  learning  not  to 
wonder  at  such  apparent  discrepancies  in  experimental  work  where 
simple  mixtures  of  two  minerals  are  compared  in  their  fusing  behavior 
with  more  complicated  mixtures  of  minerals. 


1  Italics  not  in  the  original. 

2  See  Table  XXXYI  p.  248. 


PURDY] 


PYE0-PHYS1CAL  AND  CHEMICAL  PROPERTIES. 


247 


Summarizing  these  observations  the  following  facts  appear :  First, 
Watts -has  shown  that  a  small  quantity  of  lime  toughens  a  porcelain 
mixture.  Second,  Rieke  has  shown  that  in  a.  simple  mixture  of  kaolin 
and  1  to  10  per  cent  calcium  carbonate  there  is  quite  a  large  vitrification 
range  and  slow  fusion,  while  in  mixtures  with  kaolin  containing  more 
than  10  per  cent  of  calcium  carbonate  the  body  does  not  vitrify  until 
late  and  then  rather  suddenly  fuses.  These  findings  by  Rieke  and 
Watts  agree  with  ours  in  support  of  the  assumption  that  long  vitrifica¬ 
tion  range  and  slow  fusion  generally  result  in  the  production  of  tough 
ware.  Third,  the  results  of  Bleininger,  Nauss  and  Rieke  studied  to¬ 
gether  show  very  forcibly  that  chemical  alterations  and  reactions,  may 
take  place  long  before  vitrification  and  fusion  begin.  Also,  that  each 
mixture  has  its  own  peculiar  pyro-chemical  and  physical  behavior,  and, 
as  the  mixtures  become  complicated  in  composition,  the  deductions 
drawn  from  simple  mixtures  are  found  to  hold  true  only  in  very  small 
part. 

Beyond  these  studies  in  simple  mixtures  by  Bleininger,  Nauss,  and 
Rieke,  and  the  observation  in  complicated  porcelain  mixtures,  we  have 
no  data  that  have  a  bearing  on  the  effect  of  smaller  or  larger  quantities 
of  lime  on  toughness  of  burned  wares  made  from  shales.  Contrasting 
the  work  of  Rieke  and  Nauss,  the  difficulties  that  are  encountered  when 
attempt  is  made  to  trace  the  effect  of  lime  in  such  severely  complicated 
mixtures  as  shales  are  clearly  shown. 

Effect  of  Other  Oxides  in  Ceramic  Mixtures — Practically  nothing  is 
known  concerning  the  influence  of  oxides  other  than  those  considered 
above,  except  that  in  slags  titanium  causes  increased  viscosity;  that 
potash  silicates  are  more  fluid  than  soda  silicates,  and  yet,  as  a  rule, 
less  fusible;  that  phosphoric  acid  is  expelled  from  ceramic  mixtures 
only  at  high  temperatures,  and  that,  before  expulsion  it  is  combined 
with  the  bases  forming  phosphates  that  are  analogous  to  the  silicates. 
A,  detailed  study  of  the  influence  of  the  several  oxides,  alone  and  to¬ 
gether,  on  the  fusion  of  silicate  mixtures  and  the  toughness  of  the 
burned  mixtures,  offers  a  very  fruitful  and  interesting  field  for  re¬ 
search. 


INFLUENCE  OF  SIZE  OF  GRAIN  ON  THE  FUSION  OF  CLAYS. 

Direct  evidence — According  to  Wegemann’s  microscopic  studies  given 
on  later  pages,  coarse  quartz  does  not  enter  into  the  fluxing  reactions 
even  at  cone  5.  With  a  heat  treatment  sufficient  to  fuse  cone  5  feldspar 
is  completely  fused  especially  if  mixed  with  free  silica,  and  yet  at  this 
cone  Wegemann  reports  that  the  quartz  grains  are  apparently  unaf¬ 
fected  to  any  noticeable  extent  until  cone  9  is  fused  down.  He  affirms 
that  if  any  reaction  has  taken  place  between  the  free  silica  and  feldspar, 
the  silica  must  have  been  supplied  from  what  he  terms  the  ground 
mass,  i.  e.,  the  mass  that  consists  of  particles  too  fine  to  be  distinguished 
through  the  microscope.  According  then  to  Wegemamf’s  studies,  the 
melting  feldspar  in  shales  affects  the  coarse  flint  to  but  a  slight  extent. 

Bleininger1  experimentally  determined  the  effect  of  size  of  flint  and 
feldspar  grains  on  the  rate  at  which  lime  would  decompose  them  at 
1100°  C.,  forming  silicates  that  could  be  dissolved  in  hydrochloric  acid 


248 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


Table  XXXV. 

Effect  of  Size  of  Grain  on  Extent  and  Rate  of  Combination  of  Silicia  and  Lime. 


Sizes. 

Ground 

Flint. 

150-120 

mesh. 

1120-100  100-80  80-60 
mesh.  mesh.  mesh. 

1  1  1 

60-40 

mesh. 

40-20 

mesh. 

Per  cent  residue . 

1 

28.831 

1 

63.8 

78.53;  86.52  86.27 

93.78 

96.83 

Ppr  rent  taken  into  solution . 

71.17 

1 

36.2 

1 

21.47  13.48  13.73 

l  1  1  1 

6.27 

3.17 

Table  XXXVI. 

Effect  of  Size  of  Grain  on  Extent  and  Rate  of  Combination  of  Feldspar 

and  Lime. 


Sizes. 

Ground 

Feldspar. 

150-120 

mesh. 

I 

120-100 

mesh. 

100-80 
mesh,  j 

80-60 

mesh. 

60-40  40-20 

mesh.  mesh. 

Per  cent  residue . 

3.75 

15.45 

31.00 

64.29 

79.63 

95. 721 . 

Per  cent  taken  into  solution . 

96.25 

84.55 

69.00 

35.71 

20.37 

4.28 . 

1 

and  sodium  carbonate.  The  data  he  obtained  are  given  in  the  follow¬ 
ing  tables: 

This  data,  together  with  Wegemann’s  microscopic  observations,  proves 
conclusively  that  a  variation  of  this  physical  factor — fineness  of  grain 
— has  an  influence  on  the  fusing  behavior  of  clays  that  is  as  positive,  if 
not  as  potent,  as  a  variation  in  the  quantity  of  the  oxides  of  any  of 
the  elements. 


GENERAL  ANALYSIS  OF  RESULTS. 

The  foregoing  detailed  discussion  of  the  various  elements  affecting 
the  manner  in  which  silicate  mixtures  fuse,  has  been  given  in  addition 
to  the  more  general  statements  on  pages  217  and  232  so  as  to  make 
more  plain  the  deductions  that  are  to  be  drawn  from  our  own  data. 
This  detailed  citation,  it  is  hoped,  has  clearly  demonstrated  that  our 
present  knowledge  of  the  influence  of  the  several  factors  even  in  simple 
mixtures  is  very  fragmentary  and  that  in  the  more  complex  mixtures  the 
evidence  is,  in  the  main,  either  conflicting  or  entirely  lacking.  In  the 
following  analysis  of  the  chemical  data  obtained  by  this  Survey,  and  at¬ 
tempts  to  show  a  relation  between  the  chemical  and  ph}rsical  constitution 
of  the  clays,  their  pyro-physical  behavior,  and  toughness  of  the  burned 
bricks,  liberal  assumptions  must  be  made  and  only  general  conclusions,  if 
any,  drawn. 

These  assumptions  are :  First,  Those  elements  which  are  supposed  to 
increase  the  viscosity  of  the  mass  when  fused  lengthen  the  vitrifying 
range  of  the  clay  and  increase  the  toughness  of  vitrified  wares.  Sec¬ 
ond,  Those  chemical  or  physical  factors  which  tend  to  make  the  mass 
more  fusible  or  to  hasten  the  pyro-chemical  reactions  which  result  in 
vitrification  are  detrimental  to  development  of  toughness.  Third,  That 


1  Loc.  cit.  p.  127. 


PURDY] 


PYR0-PHYS1CAL  AND  CHEMICAL  PROPERTIES. 


249 


.lime  is  detrimental  both  to  slow  fusion  and  toughness,  while  magnesia 
is  beneficial.  Fourth,  That  the  higher  the  acid  content,  or  its  equivalent, 
the  oxygen  ratio,  the  more  viscous  will  be  the  fused  ingredients  and  the 
tougher  the  burned  ware.  Fifth,  The  higher  the  proportion  of  Ab(K 
to  other  basic  oxides  the  slower  will  be  the  fusion,  the  more  viscous  the 
fused  ingredients  and  the  tougher  the  mass.  Sixth,  The  finer  the  ma¬ 
terial  of  which  clay  is  composed,  the  more  rapidly  will  it  fuse  and  the 
more  brittle  will  be  the  burned  mass. 

In  the  following  table  will  be  found  the  ratio  mentioned  in  the  fore¬ 
going  assumptions,  as  calculated  from  the  chemical  data  given  on  pages 
215  and  216.  In  the  first  column  is  the  ratio  of  CaO  to  MgO.  In  this 
ratio,  CaO  is  taken  as  unity.  In  the  second  column  is  given  the  total 
oxygen  in  the  basic  oxides  where  AhOa  is  unity. 

In  summing  up  the  oxygen  atoms,  the  iron  oxides  were  considered  as 
reported,  i.  e.,  where  FeO  is  given,  only  one  atom  of  oxygen  to  one 
atom  of  Fe,  and  where  FeaOa  is  given,  three  atoms  of  oxygen  to  two 
atoms  of  Fe  were  taken.  The  difference  between  the  value  given  in 
the  second  column  and  3  (oxygen  in  AhOa)  gives  the  factors  for  the 

'  Table  XXXVII. 


w 

I 

i  i 

1 

2 

3 

4 

5 

6 

7 

Remarks. 

( 

1 

( 

fT 

0 

(A)  Ratio  of 

CaO  to  MgO.. 

{ 

1 

1 

1 

Total  oxygen  in 
bases  . 

Oxygen  in  acid . 

(B)  Oxygen  Ratio.. 

(D)  Surface  factor, 
Purdy’s  Method. . . 

5 

c 

c 

c 

c 

0 

D 

j 

> 

L 

9 

> 

a 

o 

f 

g 

Rattler  loss  N.  B.  M. 
A.  Standard . 

K —  1 

2.39 

4.25 

13.96 

3.27 

257 

7.3 

15.82 

K—  2 . 

1.84 

4.51 

13.24 

2.92 

331 

3.24 

17.48 

Good  red  when  vitrified . 

K—  : 

J..  . 

3.36 

4.50 

13.14 

2.90 

341 

5.70 

24  89 

K—  i 

t . 

6.70 

4.3 

11.56 

2.67 

514 

8.00 

19.11 

Not  screened  when  used  at  factory 

K—  5 . 

2.95 

4.1 

12.76 

3.1 

287 

8.65 

19.36 

.  .do . 

K—  6. 

3.21 

4.2 

13.28 

3.17 

221 

11 

.5 

13.25 

K—  ' 

7 

3.10 

4.2 

11.68 

2.8 

300 

7.2 

13.89 

K" —  f 

4.47 

4.59 

12.62 

2.75 

262 

8.85 

20. '23 

K—  9 . 

2.62 

4.23 

13.72 

3.22 

195 

10.6 

14.84 

V ery  hard  coarse  clay . 

K— 10 . 

2.36 

4.39 

10.96 

2.5 

604 

2.1 

39.36 

K— 11 . 

4.20 

4.28 

9.71 

'  2.27 

339 

6.6 

28.13 

K— 12 . 

1.44 

3.95 

7.82 

1.98 

403 

2.22 

K— 13 . 

3.35 

4.34 

10.18 

2.35 

356 

4.95 

31.50 

K— 14 . 

1.36 

4.5 

15.38 

3.4 

254 

3.65 

21.24 

V ery  hard  coarse  clay . 

K— 15 . 

1.42 

4.4 

11.14 

2.52 

18.44 

F  - 

1 . 

1.94 

4.63 

12  70 

2.75 

20.84 

S  — 

1 . 

2.74 

4.11 

9.20 

2.23 

26.25 

s  —  : 

2 _ 

5.89 

4.33 

9.42 

2.17 

27.94 

R—  1 . 

4.72 

3.54 

7.92 

2.50 

397 

16.5 

16.92 

No.  2  Fire  clay . 

R-  ! 

3.99 

4.05 

11.58 

2.85 

17.80 

R—  : 

3 . 

3.05 

4.13 

9.76 

2.36 

291 

6.55 

14.80 

R—  i 

1 _ 

4.17 

4.13 

8.66 

2.10 

275 

8.45 

15.33 

H— 16 . 

0.95 

4.5 

11.56 

2.55 

H— 17 . 

1.74 

6.43 

15.92 

2.47 

H— 18 . 

0.95 

5.48 

8.26 

1.51 

444 

0.39 

H— 20 . 

1.17 

5.78 

10.36 

1.86 

553 

0.42 

H— 21 . 

0.765 

5.04 

9.5 

1.88 

783 

0.27 

H— 23 .. 

0.497 

3.97 

8.71 

2.2 

634 

O  53 

B— 11 . 

0.895 

4.20 

10.98 

2.61 

28.03 

G— II . 

1.62 

4.65 

13.28 

i  2.85 

366 

2.3 

14.98 

Good  dark  red  when  vitrified . 

H— 11 . 

0.78 

4.55 

10.18 

1  2.27 

32.97 

I— II . 

0.82 

4.67 

17.98 

i  3.85 

489 

|  1.16 

25.65 

Bright  red  when  vitrified . 

J—  II . 

1.77 

4.71 

12.58 

i  2.65 

17.14 

L— II . 

1.09 

4.47 

11.24 

2.51 

18.58 

250 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


proportion  of  oxygen  in  AbOs  to  oxygen  in  the  other  bases,  i.  e.,  (a — 3)  : 
3 :  :0  in  finxes :  0  in  AbCk  In  the  third  column  is  given  the  number 
of  atoms  of  oxygen  in  total  SiCb.  In  the  fourth  column  is  given  the 
ratio  between  oxygen  in  SiCb  to  oxygen  in  total  bases.  This  ratio  is 
known  as  the  oxygen  ratio  and  is  customarily  taken  as  the  ratio  of  the 
acids  to  the  bases.  In  the  fifth  column  is  given  the  surface  factor  rep¬ 
resenting  fineness  of  grain  by  the  writer’s  method.  In  the  sixth  col- 

A.  B,  C. 

umn  is  a  modulus  calculated  on  the  formula - =  M  where 

D. 

“A”  is  the  lime-magnesia  ratio,  “B”  the  total  oxygen  ratio,  “C”  the 
ratio  of  oxygen  in  the  Ro  bases  to  oxygen  in  AbO,  and  “D”  the  sur¬ 
face  factor  divided  by  100.  In  the  seventh  column  is  given  the  rattler 
loss  determined  on  commercially  manufactured  blocks  made  from  each 
clay. 

Deductions  Drawn  from  Table  XLI — Without  going  into  details  con¬ 
cerning  the  probable  reason  for  the  lack  of  correlation  between  the  chem¬ 
ical  and  physical  constitution  and  the  toughness  of  the  burned  ware 
as  shown  in  the  above  data,  it  is  sufficient  to  state  that  it  be  granted 
that  these  data  corroborate  those  of  Ogden,  proving  that  our  notions 
about  the  relation  of  the  chemical  and  physical  constitution  of  clays 
to  the  toughness  that  is  developed  in  burning  are  in  the  main,  if  not 
wholly,  erroneous.  Data  on  mineralogical  composition  as  obtained  by 
the  Rational  Analysis,  gave  results  that  were  still  less  easily  correlated 
with  data  on  toughness  of  the  burned  ware  than  are  those  in  the  above 
table.  Before  such  data  can  possibly  be  of  value  there  must  be  consider¬ 
ably  more  learned  concerning  the  fusing  behavior  and  the  physical  prop¬ 
erties  of  sintered  masses  of  simple  mixtures  of  minerals.  There  is  not 
much  of  any  hope  of  learning  much  concerning  these  relations  from 
data  obtained  by  any  process  of  chemical  analysis  now  used. 

THERMO-CHEMICAL  AND  PHYSICAL  CHANGES  DURING  FUSION. 

It  is  indeed  very  difficult,  if  not  impossible,  to  determine  what  the 
actual  thermo-chemical  reactions  really  are,  which  take  place  in  the 
fusion  of  the  clay  particles,  first  between  themselves,  and,  secondly, 
when  the  whole  mass  becomes  a  more  or  less  homogeneous  solution.1 
By  the  aid  of  the  microscope,  as  will  be  seen  later,  more  can  be  told  con¬ 
cerning  these  changes  in  an  unknown  mixture  of  minerals  than  bv  any 
other  means;  inferences  from  artificial  and  known  mixtures  being  of 
no  avail.  The  effect  of  thermo-chemical  reactions,  however,  can  be 
detected  by  the  changes  in  porosity  and  specific  gravity.  Because  of  our 
present  inability  to  ascertain  in  full  the  reactions  that  take  place,  it 
seems  best  to  refer  to  the  chemical  phases  of  fusion  as  “changes”  instead 
of  “reactions.” 

lProf.  G.  Tamman,  Sprechsaal  No.  35,  1904,  summarizing-  his  studies  on  sili¬ 
cates  says,  “The  volume  of  the  glass  is,  at  the  lowest  temperatures,  larger  than 
that  of  crystals.”  Mellor,  Vol.  V,  p.  78,  discusses  the  volume  changes  in  silicates 
and  cites  A.  Laurent  (Ann.  Chim.  Phys.  (2)  66,96,1837;  A.  Brongniart,  Traite  des 
Arts  Ceramiques,  1,  283,  720.  1877)  and  G.  Rose  (Pogg,  111, ‘123,  1890;  A.  R.  Day 
and  E.  S.  Shephard,  Am.  Jour.  Science,  (4)  22,  262,  1906.  Dr.  E.  Berdel  (cited 
Vol.  VII,  p.  148  A.  C.  S.  Trans.)  described  similar  physical  changes  in  the  heat¬ 
ing  of  ceramic  materials  and  bodies. 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


251 


The  greater  portion  of  the  constituents  of  onr  clays  being  mineral 
substances,  many  of  which  do  not  entirely  lose  their  identity  in  the 
burning  of  clay  wares,  it  is  most  natural  that  these  should  exhibit  in 
nature  the  same  changes  when  treated  separately  that  they  do  when 
heated  together  in  clays.  Roth1  gives  the  following  description  of  the 
physical  changes  in  minerals  on  melting: 

Table  XXXVIII. 


Mineral. 

Specific 
Gravity  of 
the  Crystal. 

Specific  Grav¬ 
ity  when 
melted  to  glass 

1Per  cent  Re¬ 
duction  in 
Spec.  Gravity. 

Quartz . 

2.663 

2.228 

16.3 

Quartz  . . 

2.65 

2.19 

17.3 

Olivine . 

3.3813 

2.8571 

15.6 

Mica . 

3.0719 

2.2405 

27.0 

Adular . 

2.561 

2.3512 

8.1 

Adular . 

2.5522 

2.33551 

8.5 

Sanidine . 

2.58 

2.381 

7.6 

Orthoclase . 

2.574 

2.328 

9.6 

Orthoclase . 

2.5883 

2.3073 

10.9 

Microcline . 

2.5393 

2.3069 

9.1 

Albite . 

2.604 

2.041 

21.9 

Oligoclase . 

2.66 

2.258 

15.1 

Oligoclase . 

2.6051 

2.3621 

9.1 

Oligoclase . 

2.6141 

2.1765 

16.7 

Labradorite . 

2.7333 

2.5673 

6.1 

Hornblende . 

3.2159 

2.8256 

12.2 

Augite . 

3.2667 

2.8035 

14.2 

Epidote . 

3.409 

2.984 

12.5 

Red  brown  garnet. . . . 

3.90 

3.05 

20.5 

Lime-iron  garnet _ 

3.838 

3.340 

25.6 

Granite . 

2.680 

2.427 

12.9 

Granite . 

2.751 

2.496 

9.3 

Hornblende  granite.. 

2.643 

2.478 

6.2 

Felsite  porphyry . 

2.576 

2.301 

10.7 

Syenite . 

2.710 

2.43 

10.3 

Quartz  diorite . 

2.667 

2.403 

9.8 

Diorite,  quartz  free.. 

2.779 

2.608 

6.3 

Gabbro . 

3.100  - 

2  664 

14.2 

1  ‘Not  in  original  table. 


Remarks. 


Average. 

Glass  compact. 

Glass  full  of  fine  bubbles. 

Glass  full  of  fine  bubbles. 

Glass  full  of  fine  bubbles, 
and  dark-colored. 

Glass  full  of  fine  bubbles. 

Glass  colorless. 

Glass  colorless. 

Full  of  fine  bubbles;  white 
glass. 

Glass  full  of  fine  bubbles. 

White  glass;  bubbly. 

Glass  full  of  bubbles. 

Glass  slightly  bubbly,  with 
black  and  white  portions 

Glass  compact. 

Glass  compact. 

Green  glass. 

Green  glass;  transparent; 
strongly  blebbed. 

Black  glass ;  opaque; 
strongly  blebbed. 

Black  glass;  opaque; 
strongly  blebbed. 

Transparent;  veryblebby; 
difficult  of  fusion. 

Glass  homogeneous;  dark 
colored. 

Glass  homogeneous;  dark 
colored. 

Black  glass;  opaque;  com¬ 
pact;  somewhat  difficult 
to  fuse. 

Black  opaque  glass;  easily 
fusible . 


The  alterations  in  the  minerals  and  rocks  above  cited  are  those  in¬ 
duced  when  they  are  changed  by  melting,  from  a  crystalline  to  an 
amorphous  condition.  Such  complete  changes  as  this  cannot  be  per¬ 
mitted  to  take  place  in  the  whole  mass  of  clay  ware  during  burning, 
and  yet,  as  will  be  shown,  the  percentage  of  decrease  in  specific  gravity 
of  many' of  our  clays  from  the  unburned  to  the  vitreous  stage  is  greater 
than  that  given  in  the  above  data.  This  being  true,  it  is  evident  that 
there  are  factors  other  than  the  alteration  of  minerals  from  the  crystal¬ 
line  to  the  amorphous  condition  that  affect  decrease  in  the  specific  grav¬ 
ity  of  clays. 

In  the  following  table  are  given  data  which  show  the  effect  of  heat 
on  physical  structure  of  briquettes  made  from  various  clays: 


l  Allegemeine  und  Chemisch3  Geologie,  Vol.  11,  p.  52. 


Table  XXXIX. 


252 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


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Table  XXXIX — Concluded, 


PURDY  ] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


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254 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


It  was  a  surprise  to  learn  that  bricks  will  decrease  in  volume  with¬ 
out  loss  of  weight,  and  at  the  same  time 'decrease  in  specific  gravity. 
Had  the  clay  been  carried  to  complete  fusion,  i.  e.,  to  a  glass,  the  de¬ 
crease  in  specific  gravity  would  have  been  credited  to  the  phenomenon 
as  in  the  case  of  minerals,  i.  e.,  the  changing  of  its  constituents  from 
crystalline  to  amorphous  forms.  But  in  the  case  of  a  clay  briquette,  a 
small  portion  of  which  enters  into,  the  fusion,  decreasing  in  specific 
gravity  before  the  minerals  have  been  rendered  amorphous,  i.  e.,  fused 
to  a  glass  or  even  before  vitrification  has  been  completed,  .cannot  be 
explained  wholly  on  this  basis.  Mr.  C.  H.  Wegemann,  of  the  geological 
department,  was,  therefore,  requested  to  make  a  microscopic  study  of 
briquettes  of  two  different  clays  burned  at  different  temperatures.  His 
report  follows : 

Notes  on  the  Microscopic  Structure  of  Certain  Paving  Brick  Clays,  at 
Various  Stages  of  Fusion. 

[By  C.  H.  Wegemann.] 

In  the  hope  of  explaining  some  of  the  phenomena  of  simultaneous  decrease 
in  volume,  porosity  and  specific  gravity  without  loss  in  weight  and  to  obtain 
some  idea  of  the  manner  in  which  fusion  takes  place  in  a  vitrifying  brick, 
microscopic  sections  were  prepared  from  briquettes  of  two  paving  brick 
clays. 


GENERAL  STRUCTURE. 

Thin  sections  of  the  briquettes  burned  at  a  low  temperature  exhibit  under 
the  microscope  a  very  fine-grained  fragmental  ground  mass,  or  matrix,  in 
which  are  imbedded  crystalline  and  other  fragments  which  were  present  in 
the  original  clay.  From  these  materials  are  developed,  at  high  temperature, 
amorphous  glasses  and  crystals. 

The  cavities  between  the  particles  of  a  brick  may  be  divided  into  two 
classes: 

(1)  Pores,  which  are  present  in  pieces  fired  at  low  temperatures,  due  to  the 
incomplete  consolidation  of  the  clay.  These  are  the  original  interstitial  spaces  of 
the  unburnt  clay. 

(2)  Blebs  or  bubbles,  which  are  formed  in  the  glass  at  higher  temperature  by 
the  liberation  and  expansion  of  gases. 

Pores  of  the  first  sort  are  of  small  size  and  irregular  outline.  As  the 
temperature  increases,  and  the  material  of  the  matrix  gradually  .fuses  into 
glass,  these  interstitial  spaces  tend  to  disappear. 

Cavities  of  the  second  sort,  which  we  may  for  convenience  designate  as 
blebs,  are  simply  gas  bubbles  in  glass.  They  are  circular  in  outline  and 
vary  greatly  in  size.  They  are  not  present  in  the  bricks  burned  at  lower 
temperatures,  but  appear  only  after  the  formation  of  considerable  glass. 

DESCRIPTION  OF  SLIDES. 

R3-14 — This  briquette  was  drawn  at  cone  3  or  about  1190°C.  The  color  is 
red.  Under  the  microscope,  the  earthy  matrix  or  ground  mass  is  dark  brown, 
the  color  being  due  to  the  presence  of  iron  oxides. 

The  mineral  fragments  are  quartz,  feldspar  and  mica,  named  in  the  order 
of  their  abundance.  They  are  angular  in  outline,  the  thin  edges  being 
sharply  defined. 

Glass  has  formed  to  some  extent  throughout  the  ground  mass  and  in  a  few 
instances  it  has  separated  out  into  clear  transparent  masses,  in  several  of 
which  blebs  appear.  The  blebs,  however,  are  so  few  and  so  small  that  the 
cavities  may  be  considered  as  made  up  almost  entirely  of  pores  of  the. first 
class.  As  estimated  under  the  microscope,  the  porosity  is  1.9  per  cent. 


PURDY]  PYRO- PHYSICAL  AND  CHEMICAL  PROPERTIES.  255 

R3-16 — Drawn  at  cone  5,  or  approximately  1230°C;  color  dark  brown. 
Under  the  microscope  the  ground  mass  appears  somewhat  denser  and  darker 
than  in  R  3-14.  The  quartz  fragments  are  apparently  unchanged.  The 
feldspar  fragments,  however,  have  disappeared.!  Mica  is  present,  but  in 
very  small  quantity. 

Glass  has  been  formed  in  considerable  amount.  It  appears  in  clear  trans¬ 
parent  areas,  often  0.1mm.  in  diameter.  In  some  of  the  glass,  needle-like 
crystals  have  begun  to  form,  but  where  free  from  these  the  glass  is  color¬ 
less.  .  This  fact  would  seem  to  indicate  that  but  little  iron  has  entered  into 
its  composition. 

As  stated  above,  fine  needle-like  crystals  are  often  present,  imbedded  in 
the  glass.  They  do  not  appear  to  have  any  definite  arrangement  with  re¬ 
spect  to  each  other,  but  occur  singly  or  in  dense  masses.  When  viewed 
singly  they  are  colorless,  but  when  seen  in  masses,  they  possess  a  greenish 
yellow  tint,  which  they  impart  to  the  glass  in  which  they  are  imbedded. 
What  the  crystals  are  was  not  determined. 

The  iron  oxides  present  in  the  matrix  have  become  segregated  into  dense 
masses,  which,  where  they  transmit  light  at  all,  show  the  red  of  hematite, 
but  no  definite  crystals  are  to  be  seen.  Pores  of  the  first  class  have  dis¬ 
appeared,  and  blebs  in  the  glass  have  become  numerous  and  large,  their 
average  diameter  being  0.066  mm.  The  estimated  pore  space  has  increased 
to  4.2  per  cent. 

R  3-18 — Drawn  at  cone  7,  or  1270°C.  The  fragments  of  quartz  appear  un¬ 
changed.  The  earthy  ground  mass  is  rapidly  fusing  into  glass,  which  has 
increased  greatly  in  amount  over  that  in  the  preceding  slide.  The  fine  needle¬ 
like  crystals  are  also  present  in  greater  number. 

Minute  crystals  of  iron  oxide  are  seen,  apparently  in  the  form  of  rhom- 
bohedrons,  having  slightly  concave  faces.  They  do  not  exceed  0.0014  mm. 
in  diameter.  The  blebs  have  an  average  diameter  of  0.1  mm.  and  the  pore 
space  has  increased  to  12.05. 

R  3-20 — Drawn  at  cone  9,  or  approximately  1310°C.  Quartz  fragments  are 
present  as  before,  but  occasionally  one  is  observed  the  edge  of  which  has 
fused  into  a  glass.  The  needle-like  crystals  are  everywhere  present  in  the 
glass,  giving  to  it  the  yellowish-green  tint  before  mentioned.  The  iron  ox¬ 
ides  appear  much  the  same  as  in  the  last  specimen.  The  blebs  are  but  little 
changed. 

R  3-22 — Drawn  at  cone  11,  or  approximately  1350°C.  The  earthy  matrix 
has  given  place  entirely  to  glass.  Quartz  particles- are  still  present,  but  thin; 
•their  edges  have  been  rounded  by  fusion. 

The  fine  needle-like  crystals  in  the  glass  have  increased  greatly  in  length, 
being  in  some  cases  0.03  mm.  long.  They  exhibit  for  the  first  time  a  marked 
tendency  to  collect  in  radiating  clusters.  Often  they  appear  to  be  attached 
to  the  corners  of  the  crystals  of  4ron  oxide.  These  latter  have  increased  in 
number  and  size,  being  0.005  mm.  in  diameter.  In  some  cases  the  individ¬ 
uals  unite,  forming  long  serrated  columns. 

,Blebs  have  increased  in  size,  their  average  diameter  being  0.128  mm. 
The  pore  space  as  estimated  from  them  is  19  per  cent. 

G  11-10 — Drawn  at  cone  02,  or  approximately  1110°C.  Color,  brick  red. 

As  in  the  R  3  series  already  described,  the  mineral  fragments  consist  of 
quartz,  feldspar  and  mica.  Very  little  glass  seems  to  have  developed  at  this 
temperature,  and  no  blebs  are  present.  .  The  pore  space  is  made  up  entirely 
of  pores  of  the  first  class,  or  those  due  to  the  imperfect  consolidation  of 
the  bricks.  The  average  diameter  of  these  pores  is  0.065  mm.,  and  the  pore 
space  as  calculated  is  2.6  per  cent. 

l  Hintze  gives  the  fusion  points  of  the  feldspar  as  ranging  from  1140C.,  in 
sanidine  to  1230  C.,  in  labradorite.  In  the  briquette  under  consideration  it  is 
evident  that  the  feldspar  has  fused  into  glass.  It  is  to  be  supposed  that  in  this 
fusing,  it  would  flux  some  of  the  quartz.  If  it  did  so,  however,  the  quartz  must 
have  been  furnished  by  the  ground  mass,  for  the  coarser  fragments  are  apparently 
not  changed  in  outline  nor  diminished  in  amount. 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


256 


G  11-12 — Drawn  at  cone  1,  or  approximately  1150°C.  Color  red. 

A  little  glass  appears,  but  no  blebs  are  seen.  The  average  size  of  pores 
is  lower  than  in  the  last  slide,  being  0.045,  but  the  pore  space  as  estimated 
runs  a  little  higher,  or  3.6  per  cent. 

It  may  be  remarked  that  in  the  slides  there  is  no  marked  increase  in 
the  pore  space,  as  temperature  increases,  up  to  the  point  where  blebs  appear. 
From  that  point  on,  pore  space  increases  rapidly. 

G  11-15 — Drawn  at  cone  3,  or  approximately  1190°C.  Color,  reddish  brown. 
Fine  needle-like  crystals  have  formed  in  the  glass.  A  few  blebs  appear, 
but  are  not  in  sufficient  number  to  affect  the  pore  space  materially.  As 
estimated,  is  is  3.2  per  cent,  while  the  average  size  of  the  pores  of  both  classes 
is  0.06  mm. 

G  11-15 — Drawn  at  cone  5,  or  approximately  1230°C.  Color,  dark  brown. 
Quartz  fragments  are  still  present,  but  the  feldspar  and  mica  have  dis¬ 
appeared.  Glass  has  formed  in  great  quantity,  being  colorless,  or  when 
acicular  crystals  are  present,  greenish  yellow.  These  crystals  are  present  in 
great  numbers  and  resemble  those  described  in  the  former  series.  Microlites 
of  iron  oxide  are  also  present,  but  have  not  yet  grouped  themselves  in  den¬ 
dritic  forms.  Pores  other  than  blebs  have  disappeared,  but  the  blebs  have 
increased  greatly  in  size,  the  average  diameter  being  0.175  mm.,  while  the 
pore  space  amounts  to  12  per  cent. 

Summary  of  Changes  Observed  at  Different  Heat  Treatments. 

Cone  12 — Quartz  and  feldspar  fragments  are  unchanged. 

But  little  glass  is  developed. 

No  blebs  have  yet  formed. 

Cone  1 — No  marked  change  has  taken  place  over  cone  12. 

Cone  3 — A  small  amount  of  glass  is  developed  from  the  ground  mass. 

A  few  blebs  appear. 

Needle-like  crystals  are  developed  in  the  glass. 

Cone  5 — Feldspar  fragments  are  fused  into  glass. 

Quartz  fragments  are  fused  into  glass. 

Blebs  increase  in  number  and  size. 

Minute  crystals  of  iron  oxide  develop. 

Cone  7 — Glass  increases  in  amount. 

Blebs  increase  in  number  and  size. 

Quartz  fragments  are  unchanged. 

Cone  9 — Quartz  fragments  begin  to  fuse  into  glass  along  their  edges. 

Cone  11 — Ground  mass  is  completely  fused  into  glass. 

Some  rounded  quartz  fragments  still  remain. 

Blebs  have  increased  remarkably  in  size  and  number. 

Microlites  are  more  numerous. 

It  should  be  borne  in  mind  that  this  is  but  a  preliminary  study.  The  num¬ 
ber  of  slides  examined  is  too  limited  to  warrant  broad  generalizations. 

Specific  Gravity,  Volume  and  Porosity  Changes  of  Clays  Studied. 

(by  r.  c.  PURDY.) 

Owing  to  the  absence  of  similar  data  on  other  clay  samples  and  the 
incompleteness  of  the  present  researches,  the  writer  has  no  definite  con¬ 
clusions  to  present  concerning  the  surprising  facts  presented  by  Mr 
Wegemann.  This  data  does,  however,  establish  the  facts  that  neither 
a  mineralogical  analysis  nor  an  ultimate  or  rational  analysis  of  clay 
will  indicate  the  nature  of  its  pyro-chemical  and  physical '  behavior. 
Indeed,  the  above  data  would  seem  to  throw  doubt  on  the  value  of  pjrro- 
chemical  and  physical  study  of  a  synthetic  mixture  of  minerals  as  a 
basis  on  which  to  interpret  the  thermal  changes  in  an  “unknown”  clay 
mixture. 

In  the  following  figures  26  and  27  are  shown  the  specific  gravity, 
volume  and  changes  in  porosity  in  the  two  clays  of  which  microscopic 
studies  were  made  by  Mr.  Wegemann.  It  will  be  seen  that  all  three- 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


257 


factors  decrease  simultaneously,  showing  that  the  increases  in  molecular 
volume  and  in  bleb  structure  is  not  sufficient  to  counteract  the  shrink¬ 
age  of  the  mass  as  a  whole,  and  is  not  to  be  accounted  for  by  the  sealing 
up  of  the  original  pores. 


- — 17  G- 


258 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


PERCENTAGE  OF  ACTUAL  POROSITY 

Fig.  27.  Curves  showing  physical  changes  in  clay  at  various  stages  of  burning. 


PURDY] 


PYRO- PHYSICAL  AND  CHEMICAL  PROPERTIES. 


259 


Differentiation  Between  Clays  on  Basis  of  Difference  in  Rate 
and  Manner  of  Decrease  in  Porosity  and  Specific  Gravity. 

INTRODUCTION. 

Importance  of  Slow  Vitrification — It  is  the  concensus  of  opinion 
among  those  who  have  given  serious  thought  to  the  vitrifying  proper¬ 
ties  of  ceramic  mixtures,  whether  natural,  as  ordinary  clay,  or  artificial, 
as  pottery  bodies,  that  those  mixtures  which  vitrify  most  slowly  and  at 
a  uniform  rate,  all  other  things  being  usual,  will  produce  the  strongest 
and  toughest  ware.  Chemical  analysis  and  synthetical  mixtures  have 
failed  to  reveal  the  happy  combination  of  minerals  or  chemical  ele¬ 
ments  that  will  produce  this  slow,  uniform  rate  of  vitrification.  A  few 
general  rules  can  be  stated  as  to  combinations  of  ingredients  required 
to  produce  tough  bodies,  but  none  of  them  can  be  applied  with  absolute 
assurance  that  they  will  operate  in  a  given  case.  With  our  present  in¬ 
formation  empirical  trials  have  to  be  resorted  to  find  the  proper  com¬ 
bination  in  each  case. 

It  is  commercially  impractical  to  alter  the  composition  of  clays  used 
for  paving  brick  manufacture  except  in  so  far  as  different  strata  permit 
of  the  use  or  rejection  of  materials  that  effect  the  character  of  the  ware. 
This  the  paving  brick  manufacture  has  learned  by  experience,  so  that 
the  composite  “dry  pan”  sample,  before  described,  is  supposed  to  repre¬ 
sent  the  best  “mix”  that  is  commericially  possible  in  a  given  case.  On 
the  supposition  that,  according  as  its  rate  of  vitrification  is  slower,  one 
clay  is  more  suited  for  vitrified  paving  brick  than  another,  and  that 
there  is  no  means  of  obtaining  information  that  bears  on  this  problem 
other  than  determining  this  very  pyro-physical  property  in  paving  brick 
elays,  clays  were  molded  into  cones  having  the  same  shape  and  dimen¬ 
sions  of  Seger  pyrometric  cones  manufactured  by  Prof.  Edward  Orton, 
Jr. 


PRELIMINARY  TRIALS. 

Manufacture  of  Test  Cones — The  clays  in  this  experiment  were  dry 
ground  in  a  mortar  to  pass  a  40  mesh  screen,  wetted  with  water  from 
the  University  mains,  wedged  thoroughly  and  molded  into  cones  with 
a  spatula  in  a  regular  cone  die  as  used  by  Orton.  On  the  upper  face  of 
each  cone  was  scratched  its  sample  and  serial  number.  After  removal 
from  the  die  the  cones  were  placed  in  a  cool  place  protected  from  drafts 
to  dry. 

Setting  of  Test  Pieces  After  Drying — One  cone  each  of  four  different 
clays  was  set  in  a  row  in  the  center  of  a  fire  clay  slab.  On  either  side 
of  the  row  of  test  cones  was  placed  a  row  of  three  standard  Seger  cones 
arranged  in  opposite  order  from  one  another.  There  were  eight  groups 
of  such  slabs  for  each  set  of  four  test  cones,  thus  allowing  eight  heat 
treatments  of  different  intensities  on  each  clay. 


260 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


The  eight  groups  with  the  standard  cones  were  as  follows: 

First  group  010-09-08. 

Second  group  07-06-05. 

Third  group  04-03-02-01. 

Fourth  group  01-1-2. 

Fifth  group  2-3-4.  • 

Sixth  group  4-5-6. 

Seventh  group  6-7-8. 

Eighth  group  8-9-10. 

Special  saggars  were  prepared,  being  3%  inches  deep  and  8  by  8 
inches  in  area,  and  having  only  three  sides.  These  saggars  were  placed 
in  four  bungs  in  the  side  down-draft  kiln  designed  by  the  writer  for  the 
ceramic  department  of  the  University  of  Illinois,  and  shown  in  Fig.  28. 
Four  of  these  special  saggars  were  placed  in  each  bung,  making  16  sag¬ 
gars  in  all. 

Burning — Four  separate  burns  were  made,  one  of  the  first  four 
groups,  one  of  the  last  four  groups,  and  a  duplicate  or  check  burn  on 
each.  , 

The  kiln  was  fired  with  coke,  in  a  manner  that  maintained  oxidizing 
conditions  throughout  the  entire  burn.  In  all  four  burns  the  fire  clay 
slabs  were  burned  to  a  clean  huff  color  showing  no  evidence  of  having 
been  subjected  at  any  time  to  reducing  influences.  Inasmuch  as  the  buff 
color  of  a  fire  clay  is  very  sensitive  to  reducing  action,  and  if  once  re¬ 
duced  the  huff  tint  is  irrevocably  bleached,  confidence  is  felt  that  in 
these  burns  we  were  successful  in  maintaining  oxidizing  conditions. 

When  a  temperature  had  been  reached  sufficient  to  cause  cone  09  to 
bend,  the  wicket  was  opened  and  the  top  saggers  from  each  of  the  four 
bungs  were  drawn  and  placed  in  the  ash  pit  of  the  kiln  where  they 
cooled  slowly.  After  placing  a  cover  over  the  exposed  cones  left  in  the 
kiln,  the  wicket  was  resealed  and  the  heat  raised  until  cone  06  was  bend¬ 
ing,  and  so  on  until  the  center  standard  cones  of  the  last  set  of  four  sag¬ 
gers  were  bending. 

By  this  scheme  of  setting  twenty-four  clays  could  be  tested  in  one 
series  of  four  burns,  there  being  in  each  draw  two  slabs  of  the  same 
group  in  each  of  the  four  saggars.  This  scheme  of  burning  was  made 
possible  by  the  fact  that  the  openings  in  the  flash  wall  leading  into  the 
firing  chamber,  and  openings  in  the  opposite  side  of  the  firing  chamber 
leading  into  the  draft  flue  caused,  with  the  down  draft,  an  equal  lateral 
distribution  of  heat.  In  no  instance  was  there  a-  failure  to  have  the 
center  test  cone  bent,  although  in  some  cases  in  the  same  draw  it  was 
bent  more  than  in  others. 

Testing  of  the  Trial  Pieces — The  cones  were  detached  from  the  slabs, 
marked  with  lead  pencil,  weighed  one  at  a  time  on  a  jolly  balance  and 
then  placed  in  clear  hydrant  water.  After  twenty-four  hours  of  satura¬ 
tion,  the  wet  and  immersed  weights  of  each  cone  were  made  and  from 
the  data  so  obtained,  their  porosity  and  apparently  specific  gravity  cal¬ 
culated. 

Difficulties  Encountered — First,  when  the  cones  were  detached  from 
the  slabs  many  broke  into  two  or  more  pieces ;  second,  a  few  of  the  cones 
were  bloated  at  the  base,  due  to  a  lack  of  oxidation ;  third,  the  cones  were 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


261 


invariably  vitrified  more  at  the  top  than  at  the  base,  thus  causing  ir¬ 
regularity  of  results  in  those  that  were  broken;  fourth,  in  those  cones 
which  had  softened  sufficiently  to  cause  them  to  bend  over,  the  pore  sys¬ 
tem  was  not  normal,  owing  to  the  strain  set  up  on  the  upper  side  and 
compression  on  the  under  side  of  the  bent  cone;  fifth,  we  were  not  sue- 


262 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


cessful  in  making  a  jolly  balance  spring  that  was  heavy  enough  to  pre¬ 
vent  the  weight  of  a  cone  stretching  it  beyond  its  elastic  limit,  give  suffi¬ 
ciently  delicate  reading. 

Data  Obtained — Although  the  test  as  a  whole  was  unsatisfactory,  it 
is  believed  that  the  data  obtained  has  a  value.  The  porosity  data  were 
ploted  on  a  diagram  as  shown  in  Figure  29.  In  this  the  linear  distance 
between  points  on  the  abscissa,  indicating  difference  in  melting  periods 
of  the  standard  cones,  is  equal  to  the  linear  distance  assigned  to  repre¬ 
sent  a  difference  of  two  per  cent  in  porosity.  The  solid  line  is  drawn 


Fig.  29.  Decrease  in  porosity  with  burning  in  terms  of  cones. 

through  points  representing  the  average  data  obtained  on  the  two  dup¬ 
licate  cones,  and  the  points  indicated  but  not  on  the  heavy  black  line 
represents  in  each  case  the  data  obtained  from  each  of  the  two  cones. 
In  case  the  data  for  one  of  the  duplicate  cones  were  missing,  as  in  K  2 
for  instance,  the  heavy  black  line  traces  the  points  representing  the  de¬ 
termined  data. 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


263 


The  dotted  line  was  drawn  through  all  possible  combinations  of  three 
points  that  were  found  to  lie  in  line  with  each  other.  In  some  cases 
there  was  only  one  light  line  and  in  others  more  than  one,  as  shown 
by  the  data  given  in  Table  XL.  The  lines  drawn  through  three  points 
lying  in  a  straight  line  are  taken  as  representing  the  slope  of  the  curve 
describing  the  change  in  porosity  with  regularly  increasing  intensity 
of  heat  treatment.  Where  there  is  a  possibility  of  more  than  one  slope, 
as  indicated  by  the  light  lines,  each  is  recorded  and  their  average  cal¬ 
culated.  The  data  in  Table  XXX  is,  therefore,  the  slope  or  tangent  of 
the  angle  that  the  light  lines  drawn  through  three  points  makes  with  the 
abscissa. 

To  obtain  this  data  a  protector  was  so  placed  on  the  line  that  the 
angle  could  be  read.  The  natural  tangent  of  the  angle  was  then  ob¬ 
tained  from  a  logarithmic  table  of  natural  functions.  Since  the  tangent 
of  the  angle  which  a.  line  makes  with  a  given  base  line,  is  the  slope  or 
inclination  of  that  line,  this  tangency  can  be  taken  as  representing  the 
rate  at  which  the  porosity  decreases  with  increasing  heat  treatment. 

In  the  following  Table  will  be  found  the  values;  first,  of  each  of  the 
tangents  of  the  angles  made  by  the  dotted  line  and  abscissa;  second,  the 
average  of  the  tangents ;  and  third,  the  rattler  loss  as  determined  on 
bricks  obtained  from  factories  using  the  several'  clays. 

Table  XL. 


Rate  of  Vitrification. 


Sample. 


1 


K-  1 
K-  2 
K-  3 
K-  4 
K-  5 


1.1708 

1.9007 

2.0323 

1.9626 

2.0732 


K-  6 
K-  7 
K-  8 
K-  9 
K— 10 
K-ll 
K— 12 
K — 13 


2.0503 

1.5900 

1.4733 

2.0965 

1.5301 

1.1041 

1.0538 

1.0265 


K — 14 
S  -  1 
S  -  2 
R-  1 

R-  2 


R-  3  . 
R—  4  . 
H— 16  . 
H— 18  . 
H — 20  . 
H— 21  . 
H-  1  . 
H — 24  . 
H — 23  . 
B-  1  . 


1.0355 

1.5697 

1.3270 

0.6208 

0.9601 

1.0538 

.9657 

1.9347 

1.2799 

2.5386 

2.4142 

1.1041 

1.1106 

2.7475 

2.5826 


2 

3 

4 

1.9840 

1.7675 

1.1504 

1.3151 

1.1508 

1.6107 

0.9708 

1.2685 

1.9486 

i .8103 
1.2167 

0.7673 

1.7321 

1.5061 

2.1283 

2.1445 

0.6128 

0.8541 

1.1237 

1.1882 

1.4641 

2.1609 

0.7954 

0.6208 

Average. 


1.1708 

1.9007 

2.0323 

1.9626 

2.0732 

2.0503 

1.7870 

1.6204 

1.5208 

1.5301 

1.3079 

1.3322 

0.9986 

1,3714 

1.5783 

1.3270 

0.6669 

1.1677 

1.2799 

1.5470 

1.7558 

1.2799 

2.5386 

1.9391 

1.6325 

1.2344 

2.7475 

2.5826 


Rattler  Loss. 


15.82 

17.48 

24.89 
19.77 
19.36 
13.25 

13.89 
20.03 
14.84 
35.74 
28.13 


31.50 

21.24 

26.25 
27.94 
16.92 

17.80 

14.80 
15.33 


29.61 


28.03 


Summary — Owing  to  the  unavoidable  inaccuracies  of  the  work  and  the 
erroneous  assumption  that  a  porosity  graph  would  trace  a  straight  line, 
the  data  given  in  the  above  Table  has  but  little  value.  Its  principal  value 
lies  in  the  developed  fact  that  as  a  rule  the  slower  the  clay  fuses  the 
tougher  appears  to  be  the  mass. 


264 


PAYING  BRICK  AND  PAYING  BRICK  CLAYS. 


[BULL.  NO.  9 


FINAL  TRIALS. 

Failing  to  solve  the  problem  at  hand  in  the  above  test,  another  and 
more  thorough  investigation  was  at  once  started,  using  not  only  a  large 
number,  but  also  a  larger  variety  of  clays.  The  manner  in  which  the 
test  pieces  for  this  latter  study  were  prepared  was  as  follows: 

Wedging — Approximately  one  pound  of  dry  clay  was  placed  on  a 
dampened  plaster-covered  table  and  sufficient  water  from  the  University 
mains  added  to  develop  the  plasticity  required  to  permit  batting  the  clay 
into  loaves.  This  was  accomplished  by  adding  water  in  small  quantities, 
and  thoroughly  working  it  into  the  clay  each  time,  until  the  mass  had  the 
desired  plasticity.  It  was  then  thoroughly  wedged  by  kneading  and 
batting  until,  on  cutting  the  mass  open,  it  appeared  to  be  compact,  i.  e. 
without  air  blebs. 

Molding — The  loaf  was  then  subdivided  into  smaller  portions,  each 
just  sufficient  to  fill  a  mold  %  inch  by  2%  by  4^4  inches.  The  slabs 
were  made  to  fill  the  mold  by  pressure  applied  in  a  screw  press.  They 
were  then  placed  in  a  miter-box  and  cut  into  briquettes  %.  inches  by  1% 
inch  by  2%  inches. 

Marking — The  laboratory  sample  number  and  a  serial  number  was 
stamped  on  each  briquette. 

Drying — The  briquettes  were  dried  in  an  open  room  at  summer  heat. 
It  had  been  found  possible  to  dry  even  the  most  tender  of  clays  in  this 
manner,  so  it  was  assumed  that  all  clays  used  in  this  test  could,  with¬ 
out  detriment,  be  subjected  to  this  treatment. 

Firing — Twenty-four  briquettes  of  each  clay  were  prepared.  The  ones 
on  which  the  serial  numbers  1  and  2  had  been  stamped  were  placed  in 
a  saggar  to  be  drawn  at  cone  010,  those  on  wdiich  the  serial  numbers  3 
and  4  were  stamped  were  placed  in  a  saggar  to  be  drawn  at  cone  08 
and  so  on — each  successive  pair  of  briquettes  of  each  clay  being  placed 
in  a  saggar  to  be  drawn  at  a  predetermined  heat  treatment  as  follows : 


Series  No.  on  briquette. 

Heat  at  which 
drawn. 

Hours  interven¬ 
ing  between 
draws. 

1,2 . 

010 

Oxidized  at  800° 

3,4 . 

08 

for  2  hours. 
From  800° C  to 
cone  010  6  hours. 

2  hours 

5,6 . 

06 

2  hours 

7,8 . 

04 

2  hours 

9,10 . 

02 

2  hours 

11,12 . 

1 

2  hours 

13,14 . 

3 

2  hours 

17,18 . 

5 

7 

2  hours 

2  hours 

19,20 . . 

9 

2  hours 

21,22 . 

11 

2  hours 

PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


265 


The  briquettes  in  the  saggars  to  be  fired  from  cones  3  to  11  were 
packed  loosely  in  coarse  white  placing-sand,  as  to  prevent  their  stick¬ 
ing  one  to  another.  Only  those  clays  known  to  be  fire  clays,  or  at 
least  sufficiently  refractory  to  withstand  severe  heat  treatment  were 
placed  in  the  saggars  to  be  drawn  at  the  higher  cones. 

The  eleven  saggars  were  placed  in  a  coke-fired,  side  down-draft  kiln 
in  a  manner  convenient  for  drawing.  The  “spy”  cones  were  centrally 
located  in  the  kiln  in  a  shield  that  protected  them  at  all  times  from 
direct  contact  with  the  flame.  When  cone  010  was  bent  over  sufficiently 
to  touch  the  plaque,  the  wicket  was  opened  enough  to  draw  the  cone 
•010  saggar,  the  wicket  replaced,  and  the  heat  slowly  raised  as  shown 
:in  the  above  table. 

Cooling — The  saggars  in  which  the  briquettes  were  placed  were  “tile 
•setters”  2  inches  deep  and  8  inches  by  8  inches  in  area.  Before  plac¬ 
ing,  another  saggar  was  inverted  over  the  one  containing  the  briquettes, 
so  that  on  drawing,  the  briquettes  were  at  no  time  exposed  to  the  rela¬ 
tively  cold  temperature  of  the  room,  except  in  one  case  of  accident. 
The  saggars  were  placed,  uncovered,  in  the  ash  pit  of  the  kiln,  where 
they  were  exposed  to  the  direct  radiation  from  the  hot  grate  bars  above. 
In  this  manenr,  the  briquettes  were  cooled  rapidly  at  first,  thus  pre¬ 
venting  the  fused  portions  in  the  briquettes  from  crystallizing  very 
much,  but  from  dull  redness  down  to  blackness  the  cooling  extended  over 
a.  considerable  period. 

The  method  of  cooling  pursued  in  this  investigation  was  not  ideal. 
The  briquettes  should  have  been  cooled  slowly  for  the  first  200°  C. 
which,  as  above  stated,  was  not  the  case.  Inasmuch  as  there  is  danger 
nf  checking  the  vitrified  briquettes  by  cooling  down  to  room  temper¬ 
ature  too  rapidly,  some  attention  should  be  given  to  the  last  as  well  as 
to  the  first  stage  of  the  cooling  period,  but  more  particularly  to  the 
first.  It  was  not  possible  to  cool  the  briquettes  under  these  ideal  con-' 
ditions,  for  the  services  of  the  kiln  were  in  demand  for  other  purposes, 
and  circumstances  did  not  permit  of  delaying  the  burning  until  such 
time  as  the  kiln  would  not  be  in  use. 

Preparation  of  Briquettes  for  Testing — When  cooled,  sand  grains  were 
found  to  be  fused  to  many  of  the  briquettes,  requiring  that  they  be 
ground  off  on  an  emery  wheel.  Care  was  taken  not  to  unduly  heat  the 
bricks  while  grinding  off  the  sand,  and  yet  as  little  water  as  possible 
was  used.  The  bricks  that  were  thus  ground  were  washed  in  distilled 
water  to  remove  all  traces  of  dirt  and  adhering  particles.  From  the 
unground  briquettes  all  adhering  particles  were  removed  by  a.  dry  stiff 
brush.  Each  briquette  was  carefully  examined  for  flaws  induced  during 
manufacture  or  cooling,  and  also  in  order  to  remove  all  adhering  por¬ 
tions,  such  as  broken  corners  that  might  have  been  detached  later  in 
the  test. 

Up  to  this  point,  all  briquettes  were  handled  together,  without  re¬ 
gard  to  sample  or  series  number,  except  as  before  indicated. 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[bull.  no.  9 


266 


In  all,  60  clays  were  prepared  for  testing,  as  above  described,  nsing 
16  to  22  briquettes  for  each.  The  briquettes  were  not  sorted,  those 
of  each  clay  being  treated  as  a  unit,  so  as  to  insure  like  conditions  at  all 
times  for  all  briquettes  of  the  same  clay. 

Drying  of  Briquettes — Briquettes  belonging  to  two  or  three  clays 
were  placed  in  a  drying  oven  and  dried  at  240°  C.  At  the  expiration 
of  four  hours  at  this  temperature,  they  were  cooled  in  dessicators  pre¬ 
paratory  to  obtaining  the  dry  weight  of  each  briquette.  The  dry  weight 
of  each  briquette  was  found  to  the  third  decimal  place  on  a  chemical 
balance. 

Saturation  of  Briquettes — After  the  dry  weights  had  been  obtained, 
the  briquettes  were  placed  in  aluminum  pans,  keeping  them  arranged  in 
the  pans  in  their  regular  serial  order.  Distilled  water  was  added  until 
only  the  upper  surface  of  each  test  piece  was  above  the  level  of  the 
water.  This  exposure  of  one  face  of  the  briquette  was  to  permit  easy 
escape  of  the  air  from  the  interior  of  the  brick,  as  it  was  being  displaced 
by  the  distilled  water.  After  standing  thus  in  water  for  18  to  24 
hours,  they  were  completely  immersed. 

After  a  total  of  48  hours  in  water,  the  briquettes  were  placed  in 
water  under  a  bell  jar,  and  -the  air  exhausted.  In  nearly  every  case, 
when  a  partial  vacuum  had  been  created,  the  air  escaped  from  the 
briquettes  at  such  a  rate  and  in  such  volumes  as  to  cause  the  water  to  ap¬ 
pear  to  be  boiling.  From  a  previous  experiment,  the  data  of  which 
are  given  in  the  following  table,  it  was  thought  that  in  the  average 
case,  fairly  complete  saturation  could  be  attained  with  15  minutes  treat¬ 
ment  in  a  partial  vacuum. 

Table  XLI. 


Showing  efficiency  of  vacum  treatment  in  effecting"  saturation. 


Sample. 

Porosity  as  de¬ 
termined  after  48 
hours’  saturation 
without  air  ex¬ 
haustion. 

Percentage  of  Gain  in  Poro¬ 
sity  at  Conclusion  of  Vacuum 
Treatment  extending  oyer 

PERIOD  OF 

5  min. 

10  min. 

15  min. 

20  min. 

S  —  2 . 

3.22 

48.1 

51.8 

57.9 

65.0 

G— 11 . 

3.3 

38.7 

42.1 

48.4 

50.6 

K—  4b  .  . 

3.93 

27.3 

35.6 

37.5 

K— 15d . 

4.22 

13.48 

14.48 

18.7 

20.8 

K— 13c . 

4.27 

44.60 

46.60 

46.6 

46.6 

K— 15c . 

4.51 

33.40 

37.50 

36.8 

38.2 

R—  4 . 

5.12 

58.2 

59.4 

61.7 

63.7 

H— 11 . 

5.29 

31.2 

35.4 

37".  6 

38.9 

R—  2 . 

6.1 

27.5 

32.2 

35.6 

36.0 

K—  6d . 

6.46 

29.9 

31.6 

35.3 

39.3 

K—  2 . 

6.55 

18.6 

20.1 

21.6 

24.3 

R—  1 . 

6.7 

10.2 

11.0 

11.0 

11.0 

B  — 11 . 

6.91 

28.0 

30.4 

31.4 

32.0 

J  —  11 . 

7.53 

11.8 

13.7 

15.7 

16.0 

I  —  11 . 

8.64 

11.8 

12.8 

14.1 

14.8 

K—  8d . 

9  06 

22.0 

23.5 

24.0 

24.9 

B  —  1  . 

9.39 

13.11 

20.3 

23.4 

K— 15b . 

19.8 

6.05 

6.22 

6.84 

7.34 

PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


267 


Each  saturated  briquette  was  in  turn  suspended  by  a  silk  thread  from 
the  beam  of  a  chemical  balance,  and  its  saturated  weight  taken,  allowing 
for  the  weight  of  the  thread.  Without  removal  from  the  balance,  a.  glass 
of  water  was  placed  on  a  bridge  spanning  the  scale  pan  in  such  a  man¬ 
ner  as  to  cause  the  briquette  to  swing  absolutely  free  but  completely 
immersed  in  the  water.  The  suspended  weight  of  the  briquette  was 
thus  taken. 

Calculations — The  percentage  of  porosity  of  each  briquette  was  cal¬ 
culated  by  the  formula : 

Wet  Weight  —  Dry  Weight 

Percentage  of  Porosity  = - • - - X  100 

Wet  Weight  —  Suspended  Weight 

Plotting  of  Results — In  the  previous  study,  that  with  clays  molded 
into  cones,  the  writer  had  arbitrarily  established  the  following  propor¬ 
tion:  Linear  length  on  ordinate,  equal  to  2  per  cent  porosity;  linear 
length  on  abscissa  equal  to  difference  of  heat  treatment  of  one  cone :  that 
is,  2:1.  This(  as  before  explained,  was  maintained  between  the  coordin¬ 
ate  factors  of  the  porosity-graphs,  so  that  the  rate  of  decrease  in  porosity 
could  be  expressed  numerically  in  terms  of  the  tangency  or  slope  of 
the  curves,  and  that  the  factors  so  obtained  would  be  comparable  one 
with  another  at  all  times. 

The  divisions  on  the  abscissas  of  the  specific  gravity  curves  are  the 
same  as  those  of  the  porosity  curves.  The  divisions  on  the  ordinate  are 
proportionally;  0.1  Sp.  Gr.  : 2  .cone  heat  ::1:2. 

Data  obtained — In  the  following  table  are  the  data  obtained  in  the 
above  study.  Data  for  a  few  more  clays  were  obtained,  but  owing  to 
their  incompleteness  they  are  not  recorded  at  this  place: 


Table  XLII. 


268  PAVING  BRICK  AND  PAVING  BRICK  CLAYS.  [bull.  no.  9 


Commercial  Possibility 
as  Judged  by  this  test. 

Paving  brick . 

.  .do . 

.  .do . 

.  .do . 

.  .do . 

.  .do . 

.  .do . 

.  .do . 

.  .do . 

..do . 

Building  brick . 

Paving  brick . 

.  .do . . 

.  .do . 

.  .do . 

Building  brick . 

Paving  brick . 

Fire  brick . 

Doubtful  brick . 

Paving  brick . 

..do . . 

Decrease  in  Porosity. 

Cone. 

t-H3C-C-t-OSt-»aSt'*U5U5  00l-IOSt-»HeOOSt-l'it- 

Per  cent. 

85.0 

90.0 

90.0 

92.0 

92.0 

84.0 

86.0 

87.0 

90.0 

93.8 

81.1 

71.00 

83.90 

93.50 

87.50 

68.2 

79.0 

28.1 

88.7 

95.0 

94.8 

Porosity  and  Specific  Gravity  of  Briquettes  Burned  at  Cones. 

tH 

4.3 

2.32 

1.41 

1.92 

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PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


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Upper  figure  in  each  case  is  percentage  of  pore  space.  Lower  figure,  specific  gravity. 
*Porosity  data  marked  with  a  star  were  calculated  by  interpolation. 


270 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


On  plotting  the  data  obtained  in  this  experiment  they  were  found  in 
most  cases  to  be  consistent,  i.  e.,  clay  used  for  particular  industries 
such  as  paving  brick,  fire  brick,  etc.,  exhibited  porosity  changes  that 
were  so  concordant  that  the  possible  commercial  use  for  each  was  pre¬ 
dicted  from  the  curves  and  in  no  case  where  the  clays  are  now  being 
employed  did  the  predicted  use  differ  from  their  present  use  as  re¬ 
ported  by  those  who  collected  the  samples. 

The  curves  in  every  instance  were  not  straight,  but  curved  so  that 
their  tangent  or  rate  of  declination  could  not  be  ascertained  without 
the  use  of  calculus.  Inasmuch  as  the  curves  did  not  describe  grad¬ 
ually  sloping  curves,  but  in  most  cases  exhibited  well  defined  lags  in 
decrease  of  porosity,  it  was  found  that  a  simple  tangent  factor  would 
not  describe  in  full  the  fusion  behavior  of  the  clays.  A  complicated 
modulus  was  devised  which  was  not  only  a  function  of  the  tangents  of 
the  sections  of  the  curves  between  points  of  lag,  but  also  the  length  of 
each  section.  Considering  the  fact,  however,  that  this  scheme  of  study¬ 
ing  the  fusion  phenomenon  is  here  first  presented,  thus  not  finding 
confirmation  by  other  experimenters,  and  since  the  modulus  does  not 
show  more  clearly  the  rate  fusion  than  does  the  curve,  no  attempt  was 
made  to  apply  the  modulus  on  the  different  types  of  clays. 

SUMMARY  OF  RESULTS  OF  TESTS. 

In  subsequent  curves  are  given  the  limits  of  the  areas  traversed  by 
the  porosity  and  specific  gravity  curves  of  the  different  types1  of  clays. 

In  Fig.  30  are  shown  the  limits  of  area  traversed  by  porosity-graphs 
of  the  fire  clays.  The  fire  clays  are  grouped  into  three  classes  according 
to  their  rate  of  decrease  in  porosity. 

Number  One  Fire  Clays — The  writers  of  Clay  Reports  have  heretofore 
failed  to  recognize  that  of  two  clays  having  similar  ultimate  chemical 
compositions  and  similar  ultimate  fusion  periods,  one  can  be  used  in 
No.  1  fire  brick,  while  the  other  would  fail  as  a  first-class  fire  brick 
material,2  and  the  one  failing  as  a  fire  brick  material  would  be  the  only 
one  that  could  with  success  be  used  in  the  stoneware  industry.  Sev¬ 
eral  examples  of  the  foregoing  were  noted  in  the  examination  of  the 
Illinois  fire  clays.  In  fact,  the  case  is  not  an  uncommon  one. 

In  fire  brick,  maintenance  of  an  open  structure  through  the  entire 
heat  range  used  in  the  various  ceramic  industries  is  essential.  On  the 
other  hand,  in  stoneware,  closeness  of  structure  at  comparatively  low 
temperatures,  or  early  vitrification  followed  by  a  long  fusion  range  is 
absolutely  required.  It  is  evident,  therefore,  that  a  classification  of  re¬ 
fractory  fire  clays  (so  called  because  they  withstand  heat  equivalent 
to  cone  27  or  more  without  failure)  should  take  account  of  this  dif- 

1.  “Types,”  as  here  used,  does  not  refer  to  geological  origin  or  age,  but  rather 
to  the  possible  commercial  use  of  the  clays. 

2  By  fire  brick  material  is  meant  what  is  known  in  trade  as  No.  1  fire  brick 
The  so-called  No.  2  fire  bricks  are,  as  a  rule,  not  worthy  of  the  distinctive  title 
“fire  brick.”  Used  in  places  exposed  to  fire  does  not  necessarily  make  a  brick  a 
fire  brick,  for,  if  this  were  so,  the  comparatively  fusible  Chicago  brick  placed 
in  the  arches  of  their  scove  kilns  would  have  to  be  called  “fire  brick.” 


PURDY] 


PYRO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


271 


Fig.  30.  Differentiation  of  fire  clays  on  basis  of  porosity  changes. 

ference  in  their  manner  of  fusion.  This  essential  difference  in  the  be¬ 
havior  of  fire  clays  is  recognized  in  a  tentative  scheme  of  classifica¬ 
tion  presented  by  the  present  writer  and  Mr.  Moore.1 

It  will  be  noted  from  Fig.  30  that  these  clays  show  comparatively 
little  decrease  in  porosity  from  cone  010  to  cone  11.  This  decrease 
averages  from  7  to  15  per  cent  of  the  initial  porosity  and  in  no  case 
does  it  exceed  17  per  cent. 

The  specific  gravity,2  as  shown  in  Fig.  31  remains  fairly  constant 
from  cone  010  to  cone  3  and  then,  even  in  the  purest  clays,  it  begins 
to  decrease  slightly.  This  decrease  in  specific  gravity  in  the  No.  1 
fire  clays,  even  when  the  porosity  remains  very  high,  is  considered  as 
evidence  of  the  influence  of  the  adsorbed  or  cementing  salts  which, 
while  constituting  but  a  very  small  part  by  weight  of  the  whole,  are 
nevertheless  potent  factors  in  causing  fusion. 

1  Trans.  Am.  Soc.,  Vol.  IX,  pp.  239. 

2  The  specific  gravity  here  referred  to  is  the  specific  gravity  of  that  portion  of 
a  saturated  brick  not  occupied  by  water.  Inasmuch  as  this  water  impermeable 
mass  very  often,  in  fact,  in  the  case  of  impure  clays  generally  does  contain  in¬ 
closed  or  sealed  pores  known  as  blebs,  the  specific  gravity  so  obtained  cannot  be 
the  actual  specific  gravity  of  the  material  of  which  this  water  impermeable  portion 
consists.  ''The  true  specific  gravity  of  the  material  can  be  obtained  by  crushing  the 
brick  to  fine  powder,  thus  eliminating  the  sealed  pores,  and  then  determining  the 
specific  gravity  of  the  powder  in  a  specific  gravity  bottle  as  before  described.  For 
this  reason,  the  writer  has  classified  specific  gravities  under  three  heads:  First, 
false  specific  gravity,  or  the  weight  per  unit  volume  of  the  whole  brick ;  Second, 
apparent  specific  gravity,  or  the  weight  per  unit  volume  of  the  water-impermeable 
mass;  Third,  true  specific  gravity,  or  weight  per  unit  volume  of  solid  material  in 
the  water-impermeable  mass. 


272 


PAVING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  & 


Fig.  31.  Curves  showing  changes  in  specific  gravity  of  fire  clays  with  pro¬ 
gressive  intensity  of  heat  treatment. 

dumber  Two  Fire  Clays — It  will  be  noted  that  while  the  decrease  in 
specific  gravity  of  this  group  of  clays  is  about  the  same  as  that  shown 
in  the  No.  1  fire  clays,  the  porosity  shows  a  much  larger  decrease.  The 
■earthy  vitrification  and  slow  fusion  is  quite  pronounced  in  this  group, 
permitting  their  use  in  the  paving  brick,  sewer  pipe,  stoneware  and 
■terra-cotta  industries,  but  not  in  the  manufacture  of  No.  1  fire  brick. 

Number  Three  Fire  Clays — In  Figs.  27. and  28  are  shown  the  limiting 
area  of  porosity  and  specific  gravity  curves  of  a  class  of  clays  which, 
in  the  judgment  of  the  writer,  ought  to  be  put  in  a  different  category 
from  the  preceding  group,  or  number  two  fire  clays.  Heretofore,  both 
have  been  classed  together  indiscriminately  in  ceramic  and  geological 


PURDY] 


PYEO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


273 


■literature,  as  number  two  fire  clays,  but  they  are  not  the  same.  Clays 
of  this  class  differ  from  the  No.  1  and  No.  2  fire  clays,  in  that  they 
•seldom  have  a  fusion  point  exceeding  cone  16  or  17,  fuse  in  a  very  irreg¬ 
ular  manner,  and  exhibit  a  much  larger  decrease  in  specific  gravity 
owing  probably  to  the  presence  of  iron  in  nodular  form  as  sulphides  or 
carbonates. 

Fire  Clays  in  General — These  conclusions  may  be  summarized  as  fol¬ 
lows:  First,  While  all  the  types  of  fire  clays  here  tested  maintained 
•the  same  range  in  porosity  up  to  cone  010,  there  is  a  marked  differ¬ 
entiation  of  each  at  cone  08.  Second,  From  cone  08  ta  about  cone  1 
•the  No.  2  and  No.  3  fire  clays  traverse  a  common  area,  but  at  cone  1 
•the  No.  3  type  begins  to  fuse  more  rapidly,  until  when  cone  7  is 
•reached,  the  No.  3  fire  clays  have  fused  sufficiently  to  be  wholly  differ¬ 
entiated  from  the  No.  2.  Third,  Since  the  porosity  curves  in  Fig.  27 
are  composite  curves  showing  the  limits  of  variation  in  the  few  clays 
tested,  it  is  possible  that  broader  limits  will  be  determined  when  more 
and  a  larger  variety  of  clays  are  tested,  yet  the  data  here  presented  are 
sufficient  to  demonstrate  that  where  chemical  analysis  and  fusion  period 
determinations  have  failed,  this  method  of  differentiation  has  proved 
successful.  Fourth,  Differentiation  of  firer  clays  on  the  basis  of  specific 
changes  will  hardly  be  possible  on  account  of  the  limited  differences 
between  the  areas  traversed  by  the  specific  gravity  curves  of  each  type  of 
•fire  clay,  yet  as  is  shown  in  Fig.  28  the  specific  gravity  curves  parallel 
and  diverge  from  one  another  at  about  the  same  temperatures  as  do 
the  porosity  curves  in  Fig.  27. 

Chemical  analysis  and  ^points  of  fusion  of  a  few  of  the  fire  clays  from 
which  curves  were  drawn  are  as  follows: 

Table  XLIII. 


No.  1 — Fire  Clays. 


Sample 

Number. 

Moisture. 

Volatile 

Matter. 

Si02 

. 

ai2o3 

Fe203 

TiOz 

Total. 

Fusion 

point. 

H— 24  .. . . 

0.6 

4.63 

76.10 

15.31 

1.10 

1.31 

99.06 

30 

V-ll .... 

1.74 

10.28 

56.28 

26.68 

3.24 

1.29 

99.50 

Not  reached. .. 

F— 18 .... 

0.84 

6.66 

66.88 

21.87 

2.23 

1.18 

99.86 

29 

F— 19 .... 

1.19 

6.31 

68.12 

20.08 

1.76 

1.16 

98.62 

31 

Table  XLIV. 
No.  2 — Fire  Clays. 


Sample 

Number. 

Moisture. 

V  olatile 
Matter. 

Si02 

ai2o3 

Fe2Oa 

Ti02 

Total. 

[Fusion 

point. 

V — 4  .... 

2.37 

8.84 

54.80 

29.44 

1.70 

0.82 

97.97 

Not  reached. .. 

K-12.,.. 

0.60 

10.09 

54.37 

23.61 

6.14 

*5.97 

100.78 

..do . 

l  Total  fluxes  TiO,  was  not  determined  in  K-12. 


—18  G 


274 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


Chemical  analyses  were  not  made  of  all  the  clays  of  the  Xo.  1  and 
Xo.  2  type  and  none  of  the  Xo.  3.  From  the  few  that  were  made,  how¬ 
ever,  it  is  evident  that  refractoriness  and  slow  fusion  are  not  always  de¬ 
pendent  upon  the  proportional  content  of  alumina  and  silica,  for  the 
two  Xo.  2  fire  clays  have  on  the  average  higher  AhOs  and  lower  SiO* 
content  that  the  Xo.  1  fire  clays.  This  is  directly  contrary  to  our  past 
teachings  and  contrary  to  what  might  be  expected  from  Segar’s  AbCh- 
SiCb  curve,  as  shown  in  figure  19,  page  208. 

Paving  and  Building  Brick  j Clays — The  standardization  of  tests  for 
first-class  paving  brick  clays  has  been  and  perhaps  will  be  for  some  time 
the  subject  of  consideration  by  ceramic  investigators.  The  pyro-physi- 
cal  and  chemical  tests  here  reported  can  be  said  to  give  negative  rather 
than  positive  information,  in  that  they  very  effectively  differentiate  the 
clays  they  cannot  from  those  that  may,  be  utilized  in  paving  brick  man¬ 
ufacture.  J udging  from  the  results  so  far  obtained,  they  fail,  how¬ 
ever,  to  differentiate  the  paving  brick  clays  one  from  another  in  re¬ 
gard  to  their  comparative  quality.  For  example,  we  have  not  been 
able  to  distinguish  by  these  tests  between  the  clays  of  14  per  cent  and  the 
24  per  cent  type,  measured  in  per  cents  of  loss  in  the  rattler  test,  nor 
between  the  clays  that  preserve  their  maximum  strength  through  a 
wide  heat  range  and  those  which  attain  and  preserve  their  maximum 
strength  only  within  a  very  narrow  heat  range. 

The  cause  of  failure  of  the  pyro-chemical  studies  in  this  respect  is, 
no  doubt,  to  be  found  in  the  fact  that  inherent  strength  is  not  wholly 
a  function  of  rate  of  vitrification  or  development  of  vesicular  structure. 
As  shown  in  earlier  pages,  physical  tests  on  the  raw  clays  failed  to  dif¬ 
ferentiate  paving  from  building  brick  clays.  The  pyro-chemical  studies 
here  reported  are  the  only  ones  that  give  any  clue  to  cause  of  toughness 
or  strength  of  the  burned  ware. 

Pyro-chemical  studies  similar  to  those  here  outlined,  together  with  a 
determination  of  the  maximum  strength  and  the  range  of  temperature  in 
which  this  maximum  strength  is  developed,  would  enable  the  observer 
to  properly  classify  and  differentiate  paving  brick  clays.  This,  however, 
amounts  to  a  sub-classification  of  the  paving  clays  on  a  basis  different 
from  that  of  the  main  sub-division. 

The  striking  differences  between  the  building  and  paving  brick  clays 
are  apparent  from  figures  32  and  33.  Earlier  vitrification,  irregularity 
in  decrease  of  porosity  and  specific  gravity,  apparently  larger  quantity 
of  vessicular  glass  formed  within  the. mass,  or  at  least  a  more  notable 
bloating,  due,  to  volatilization  of  certain  constituents,  probably  the  sol¬ 
uble  and  adsorbed  salts,  are  the  distinguishing  features  of  the  strictly 
building  brick  class. 

Sufficient  evidence  is  at  hand  to  warrant  the  statement  that  any  clay 
which  vitrifies  to  a  porosity  of  2  or  3  per  cent  before  cone  5  is  reached, 
in  the  heat  treatment  prescribed  in  this  method  of  burning  test  pieces, 


PURDY] 


PYEO-PHYSICAL  AND  CHEMICAL  PROPERTIES. 


275 


will  be  too  brittle  for  use  as  paving  brick  material,  no  matter  how  little 
vesicular  structure  is  developed.  The  fact  is,  however,  that  it  will  be  a 
rare  case  in  which  vesticular  structure  is  not  strongly  developed  if  the 
clay  shows  an  early1  and  rapid  rate  of  vitrification. 

In  figures  32  and  33  are  shown  the  upper  and  lower  limits  of  areas 
that  were  traversed  respectively  by  the  porosity  and  specific  gravity 
curves  of.  clays  that  either  are  being  or  can  be  used  for  the  purposes  in¬ 
dicated  in  the  figures. 


Fig.  32.  Curves  showing-  changes  in  porosity  of  paving  and  building  brick 
clays  with  progressive  intensity  of  heat  treatment. 


1  The  use  of  the  comparative  terms  “early”  and  “rapid”  in  reference  to  this 
type  of  clays,  in  contrast  to  their  relative  use  in  regard  to  fire  clays,  is  best  illus¬ 
trated  by  reference  to  the  curves. 


276 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[BULL.  NO.  9 


Fig.  33.  Curve  showing  changes  in  specific  gravity  of  paving  and  building 
brick  clays  with  progressive  intensity  of  heat  treatment. 

The  boundary  limits  shown  in  these  figures  are  those  obtained  in 
these  tests,  and,  therefore,  may  not  show  exactly  the  true  limits  of  the 
several  areas.  They  indicate,  however,  approximately  the  relative  man¬ 
ner  in  which  the  clays  used  for  the  several  industries  behave  in  fusiilg. 

All  clays  used  for  paving  and  sewer  brick  may  be  used  for  building 
brick,  but  what  are  here  defined  as  strictly  building  brick  clays  cannot 
be  used  for  paving  or  sewer  brick.  All  paving  brick  clays  can  be  used 
in  the  manufacture  of  sewer  brick,  but  the  sewer  brick  clays  cannot 
be  used  for  paving  brick.  The  points  of  differentiation  are;  first,  the 
paving  brick  clay  fuses  more  slowly  and  decreases  less  in  specific  gravity ; 


PURDY]  PYRO- PHYSICAL  AND  CHEMICAL  PROPERTIES.  277 

second,  the  sewer  (and  side  walk)  brick  clay  fuse  more  rapidly  bnt 
maintain  their  shape  through  a  considerable  range  of  heat  treatment 
before  failing;  third,  those  clays  which  are  fit  only  for  building  brick 
vitrify  rapidly  and  fail  as  soon  as,  or  before  they  are  completely  vitri¬ 
fied.  The  sewer  brick  clays  can  be  brought  with  safety  to  complete 
vitrification  without  much  danger  of  loss  except  perhaps  from  “kiln 
marking”  while  those  clays  which  are  fit  only  for  building  brick  bloat 
and  become  spongy  as  well  as  soft  almost  as  soon  as  vitrification  takes 
place. 

Since  the  tracing  of  the  porosity  curves  through  the  upper  or  paving . 
brick  clay  area  does  not  necessarily  signify  that  they  are  good  for. paving 
brick  manufacture,  the  lower  limit  may  appear  to  be  superfluous.  It 
remains  a  fact,  however,  that,  according  to  the  tests  here .  reported,  a 
clay  must  have  its  porosity  curve  confined  within  the  limiting  bound¬ 
aries  shown  in  order  to  develop  the  required  toughness.  So  far  as  ex¬ 
perience  with  the  Illinois  clays  is  concerned,  the  curves  for  porosity 
and  specific  gravity  in  figure  29  and  30  respectively,  denote  quite  rig¬ 
idly  the  allowable  variation  in  rate  of  decrease  in  porosity  and  specific 
gravity. 


GENERAL  CONCLUSION. 

In  the  preceding  discussions  of  physical,  chemical,  and  pyro-physical 
and  chemical  properties  of  clays  all  of  the  relations  between  these  prop¬ 
erties  that  were  known  or  observed  have  been  shown.  A  review  of  these 
discussions  reveals  the  following  as  being  the  most  important. 

1.  Measurement  of  some  of  the  properties  failed  to  give  results  that 
show  the  factors  which  affect  them  or  which  are  involved  with  them. 
This  was  made  plain  in  case  of  the  “individual  grains”  as  obtained  by 
mechanical  analysis.  We  have  seen  that,  the  methods  universally  em¬ 
ployed  to  effect  the  physical  disintegration  of  clay  are  not  sufficiently 
intensive  to  produce  complete  disintegration.  It  has  also  been  demon¬ 
strated  that  the  grains  or  particles  so  obtained  do  not  usually  consist 
of  one  mineral  substance.  As  a  consequence  of  this  cementation  of 
smaller  particles  of  different  substances  into  bundles  or  groups,  any  in¬ 
ference  or  conclusion  based  on  fineness  of  grain  cannot  be  very  general 
in  application. 

2.  Ultimate  analysis  or  gross  rational  analysis  of  clay  cannot  reveal 
qualities  that  affect  either  the  “working”  or  “burning”  properties. 

3.  Either  ultimate  or  rational  analysis  of  the  several  groups  of  grains 
may  reveal  some  important  relation  of  constitution  to  manifested  prop¬ 
erties.  This,  however,  remains  to  be  demonstrated.  It  can  be  said 
however,  that  such  determinations  will  not  likely  become  “commercial” 
tests  of  clays.  On  the  other  hand,  however,  it  seems  certain  that  they 
will  be  valuable  for  research  purposes. 

4.  Vitrification  behavior,  rate  of  fusion  or  toughness  of  bricks,  do 
not  seem  to  depend  within  any  but  very  wide  limits  or  in  any  traceable 
manner  upon  chemical  or  mineralogical  constitution  of  clay. 

5.  No  combinations  of  physical  and  chemical  properties  can  be  said 
to  be  essential  to  clays  from  which  first-class  paving  brick  may  be 
manufactured. 


278 


PAYING  BRICK  AND  PAVING  BRICK  CLAYS. 


[bull.  no.  9 


6.  The  most  satisfactory  tests  tried  or  developed  during  the  course 
of  these  researches  for  distinguishing  between  clays  on  the  basis  of  their 
commercial  availability  are  rate  of  decrease  in  porosity  and  specific 
gravity.  While  even  these  tests,  so  far  as  can  be  judged  by  our  results, 
do  not  make  an  absolute  discrimination,  the  discussions  and  curves  here 
given  make  plain  the  fact  that  such  tests  are  the  most  serviceable  of  any 
so  Tar  developed..  The  other  tests  have  special  uses  and  are  not  to  be 
entirely  condemned. 

7.  Toughness  of  brick  does  not  bear  a  consistent  relation  to  degree 
or  range  of  vitrification.  Each  clay  has  its  own  peculiar  range  and  de¬ 
gree  of  vitrification  at  which  its  maximum  toughness  is  developed.  In 

.  some  clays  this  range  is  very  small  and  in  some  quite  large.  In  some 
clays  maximum  toughness  is  attained  when  the  brick  still  shows  an  ab¬ 
sorption  of  8  or  12  per  cent  and  in  others  not  until  the  absorption  has 
been  decreased  to  2  or  4  per  cent.  No  tests  other  than  the  “rattler  test” 
on  full  size  brick  which  have  been  burned  with  different  intensity  of  heat 
treatment  have  brought  out  data  which  bear  on  this  peculiarity  of  clays. 

8.  The  pyro-physical  studies  which  have  been  described  suggest  a 
series  of  determinations  which  should  be  more  valuable  in  that  they 
ought  to  reveal  the  cause  for  this  want  of  correlation  of  toughness  and 
vitrification  behavior.  The  series  of  determinations  referred  to  is  that 
of  the  volume  changes  which  take  place  with  increasing  intensity  of 
heat  treatment.1 

The  volume  changes  which  are  important  are: 

(a)  exterior  volume  of  brick. 

(b)  volume  of  skeleton  of  brick. 

(c)  volume  of  open  pores. 

(d)  volume  of  sealed  pores. 


1  See  Trans.  Am.  Cer.  Soc.,  Vol.  X. 


} 


